CSS3 – Animation

CSS transitions, covered in the previous chapter, enabled simple
animations. With transitions, an element’s properties change from the
values set in one rule to the values set in a different rule as the element changes state over time, instead of changing instantly. With
CSS transitions, the start and end states of property values are
controlled by existing property values, and provide little control over
how things change over time.

CSS animations are similar to transitions in that values of CSS
properties change over time, but provide much more control over how those changes happen. Specifically, CSS keyframe animations let us decide if and how an animation repeats, give us granular control over what happens throughout the animation, and more. While transitions trigger implicit property values changes, animations
are explicitly executed when animation keyframe properties are applied.

With CSS animations, you can change property values that are not part of
the set pre- or post-state of an element. The property values set on the
animated element don’t necessarily have to be part of the animation
progression. For example, with transitions, going from black to white
will only animate through various shades of gray. With animation, that same
element doesn’t have to be black or white or even in-between shades of
gray during the animation.

While you can transition through shades of gray,
you could instead turn the element yellow, then animate from
yellow to orange. Alternatively, you could animate through various colors,
starting with black and ending with white, but progressing through the entire
rainbow along the way. This chapter will explore how keyframe animation works.

Tip

Look for the Play symbol to know when an online example is available. All of the examples in this chapter can be found at https://meyerweb.github.io/csstdg4figs/18-animations.

Defining Keyframes

To animate an element, we need to set the name of a keyframe animation; to do
that, we need a named keyframe animation. Our first step is to define
this reusable CSS keyframe animation using the @keyframes at-rule,
giving our animation a name. The name we
define will then be used to attach
this particular animation to elements or pseudo-elements.

A @keyframes at-rule includes the animation identifier, or name, and
one or more keyframe blocks. Each keyframe block includes one or more
keyframe selectors with declaration blocks of property-value pairs.
The entire @keyframes at-rule specifies the behavior of a single
full iteration of the animation. The animation can iterate zero or more
times, depending mainly on the animation-iteration-count property
value, which we’ll discuss in “Declaring Animation Iterations”.

Each keyframe block includes one or more keyframe selectors. The keyframe selectors are percentage-of-time positions along the duration of the animation; they are declared either as percentages, or with the keywords from or to. Here’s the generic structure of an animation:

@keyframes animation_identifier {
  keyframe_selector {
    property: value;
    property: value;
  }
  keyframe_selector {
    property: value;
    property: value;
  }
}

and here are a couple of basic examples:

@keyframes fadeout {
    from {
        opacity: 1;
    }
    to {
        opacity: 0;
    }
}

@keyframes color-pop {
    0% {
        color: black;
        background-color: white;
    }
    33% { /* one-third of the way through the animation */
        color: gray;
        background-color: yellow;
    }
    100% {
        color: white;
        background-color: orange;
    }
}

The first set of keyframes shown takes an element, sets its opacity to 1 (fully opaque), and animates it to 0 opacity (fully transparent). The second keyframe set sets an element’s foreground to black and its background to white, then animates the foreground black to gray and then white, and the background white to yellow and then orange.

Note that the keyframes don’t say how long this should take—that’s handled by a CSS property dedicated to the purpose. Instead they say “go from this state to that state” or “hit these various states at these percentage points of the total animation.” That’s why keyframe selectors are always percentages, or from and to. If you try to fill time values (like 1.5s) into your keyframe selectors, you’ll render them invalid.

Setting Up Keyframe Animations

To create an animation, you start with the @keyframes, add an animation name, and drop in curly brackets to encompass the actual keyframes you’re defining. It’s a lot like a media query at this point, if you’re familiar with those (see Chapter 20).

Within the opening and closing curly brackets, you
include a series of keyframe selectors with blocks of CSS that
declare the properties you want to animate. Once the keyframes are defined,
you “attach” the animation to an element using the animation-name property. We’ll discuss that property shortly, in “Naming Animations”.

Start with the at-rule declaration, followed by the animation name and
brackets:

@keyframes nameOfAnimation {
...
}

Naming Your Animation

The name, which you create, is an identifier or a string. Originally, the keyframe names had to be an identifier, but both the specification and the browsers also support quoted strings.

Identifiers are unquoted and have specific rules. You can use any
characters a-z, A-Z, and 0-9, the hyphen (-), underscore (_), and any ISO
10646 character U+00A0 and higher. ISO 10646 is the universal character
set; this means you can use any character in the Unicode standard that
matches the regular expression [-_a-zA-Z0-9\u00A0-\u10FFFF]. The identifier can’t start with
a digit (0-9) or two hyphens. One hyphen is fine, as long as it is
not followed by a digit—unless you escape the digit or hyphen with a backslash.

If you include any escape characters within your animation name, make
sure to escape them with a backslash (\). For example, Q&A! must be
written as Q\&A\!. ✎ can be left as ✎ (no, that’s not a typo),
and is a valid name as well. But if you’re going to use any
keyboard characters that aren’t letters or digits in an identifier, like
!, @, #, $, and so on, escape them with a backslash.

Also, don’t use any of the keywords covered in this chapter as the name
of your animation. For example, possible values for the various
animation properties we’ll be covering later in the chapter include
none, paused, running, infinite, backwards, and forwards. Using an animation property keyword, while not prohibited
by the spec, will likely break your animation when using the animation
shorthand property (discussed in “Bringing It All Together”). So, while you can legally name your
animation paused (or another keyword,) I strongly recommend against it.

Keyframe Selectors

Keyframe selectors provide points during our animation where we set the
values of the properties we want to animate. In defining animations, we
dictate the values we want properties to have at a specific percentage of
the way through the animations. If you want a value at the start of the
animation, you declare it at the 0% mark. If you want a different value
at the end of the animation, you declare the property value at the 100%
mark. If you want a value a third of the way through the animation, you
declare it at the 33% mark. These marks are defined with keyframe
selectors.

Keyframe selectors consist of a comma-separated list of one or more
percentage values or the keywords from or to. The keyword from is
equal to 0%. The keyword to equals 100%. The keyframe selectors are used to
specify the percentage along the duration of the animation the keyframe
represents. The keyframe itself is specified by the block of property
values declared on the selector. The % unit must be used on percentage
values. In other words, 0 is invalid as a keyframe selector:

@keyframes W {
    from {
      left: 0;
      top: 0;
    }
    25%, 75% {
      top: 100%;
    }
    50% {
      top: 50%;
    }
    to {
      left: 100%;
      top: 0;
    }
}

This @keyframes animation, named W, when attached to a
non-statically positioned element, would move that element along a W-shaped path. W has five keyframes: one each at the 0%, 25%, 50%, 75%, and 100%
marks. The from is the 0% mark. The to is the 100% mark.

As the property values we set for the 25% and 75% mark are the same,
we can put the two keyframe selectors together as a comma-separated list. This is very similar to regular selectors, where you can comma-group several together. Whether you
keep those selectors on one line (as in the example) or put each selector on its
own line is up to your personal preference. The following is just as valid as what we saw in the previous code:

    25%,
    75% {
      top: 100%;
    }

Note that selectors do not need to be listed in ascending order. In the preceding example, we have the 25% and 75% on the same line, with the 50% mark coming after that declaration. For legibility, it is highly
encouraged to progress from the 0% to the 100% mark. However, as
demonstrated by the 75% keyframe in this example, it is
not required. You could define your keyframes with the last first and the first last, or scramble them up randomly, or whatever works for you.

Omitting from and to Values

If a 0% or from keyframe is not specified, then the user agent
(browser) constructs a 0% keyframe using the original values of the
properties being animated, as if the 0% keyframe were declared with the
same property values that impact the element when no animation was
applied, unless another animation applied to that element is currently animating the same property (see the upcoming section “Naming Animations” for details). Similarly, if the 100% or to keyframe is not defined and no other animations are being applied, the
browser creates a faux 100% keyframe using the value the element would
have had if no animation had been set on it.

Assuming we have a background-color change animation:

@keyframes change_bgcolor {
    45% { background-color: green; }
    55% { background-color: blue; }
}

And the element originally had background-color: red set on it, it
would be as if the animation were:

@keyframes change_bgcolor {
    0%   { background-color: red; }
    45%  { background-color: green; }
    55%  { background-color: blue; }
    100% { background-color: red; }
}

Or, remembering that we can include multiple identical keyframes as a
comma-separated list, this faux animation also could be written
as:

@keyframes change_bgcolor {
    0%,
    100% { background-color: red; }
    45%  { background-color: green; }
    55%  { background-color: blue; }
}

Note the background-color: red; declarations are not actually part of
the keyframe animation. If the background color were set to yellow in
the element’s default state, the 0% and 100% marks would display a
yellow background, animating into green, then blue, then back to yellow
as the animation progressed:

@keyframes change_bgcolor {
    0%, 100% { background-color: yellow; }
    45% { background-color: green; }
    55% { background-color: blue; }
}

We can include this change_bgcolor animation on many elements, and the
perceived animation will differ based on the element’s value for the
background-color property in the non-animated state.

Although we’ve been using exclusively integer values for our percentages, non-integer percentage values, such as 33.33%, are perfectly
valid. Negative percentages, values greater than 100%, and values that aren’t
otherwise percentages or the keywords to or from are invalid and
will be ignored.

Repeating Keyframe Properties

In the original -webkit- implementation of animation, each keyframe
could only be declared once: if declared more than once, only the last
declaration would be applied, and the previous keyframe selector block
was ignored. This has been updated. Now, similar to the rest of CSS, the
values in the keyframe declaration blocks with identical keyframe values
cascade. In the standard (nonprefixed) syntax, the preceding W animation
can be written with the to, or 100%, declared twice, overriding the
value of the left property:

@keyframes W {
  from, to {
    top: 0;
    left: 0;
  }
  25%, 75% {
    top: 100%;
  }
  50% {
    top: 50%;
  }
  to {
    left: 100%;
  }
}

Notice how to is declared along with from as
keyframe selectors for the first code block? That sets both top and left for the to keyframe. Then, the left value is
overridden for the to in the last keyframe block.

Animatable Properties

It’s worth taking a moment to talk about the fact that not all properties are animatable. Within an animation’s keyframe, if you list a property that can’t be animated, it’s just ignored. (For that matter, so are properties and values that he browser doesn’t recognize at all, the same as any other part of CSS.)

There is a comprehensive list of animatable properties in Appendix A. We’ve also indicated whether properties can or can’t be animated throughout the rest of this book, as the properties are defined.

Note

The animation-timing-function, described in greater detail
in “Changing the Internal Timing of Animations”, while not an animatable property, is not ignored. If you include
the animation-timing-function as a keyframe style rule within a
keyframe selector block, the timing function of the
properties within that block will change to the declared timing function
when the animation moves to the next keyframe.

If an animation is set between two property values that don’t have a calculable
midpoint, the results may not be what you expect. The property will not
animate correctly—or at all. For example, you shouldn’t declare an
element’s height to animate between height: auto and
height: 300px. There is no midpoint between auto and 300px. The
element may still animate, but different browsers handle this
differently: Firefox does not animate the element; Safari may animate as
if auto is equal to 0; and both Opera and Chrome currently jump from
the preanimated state to the postanimated state halfway through the
animation, which may or may not be at the 50% keyframe selector,
depending on the value of the animation-timing-function. In other
words, different browsers behave differently for different properties
when there is no midpoint, so you can’t be sure you will get your
expected outcome.

The behavior of your animation will be most predictable if you declare
both a 0% and a 100% value for every property you animate.

For example, if you declare border-radius: 50%; in your animation, you
may want to declare border-radius: 0; as well, because there is no
midpoint between none and anything: the default value of
border-radius is none, not 0. Consider the difference in the following two animations:

@keyframes round {
    100% {
        border-radius: 50%;
    }
}
@keyframes square_to_round {
    0% {
        border-radius: 0%;
    }
    100% {
        border-radius: 50%;
    }
}

The round animation will animate an element from the original border-radius value of that element to border-radius: 50% over
the duration of the animation. The square_to_round animation will animate an element from border-radius: 0% to border-radius: 50% over
the duration of the animation. If the element starts out with square corners, then the two will have exactly the same effect. But if it starts out with rounded corners, then square_to_round will jump to rectangular corners before it starts
animating. This might not be what you want. Sometimes you can omit a from or to keyframe, using the element’s non-animated property values to your advantage.

The best way to resolve this
issue is to use the round animation instead of square_to_round, making sure any element that gets animated with the round keyframe
animation has its border-radius explicitly set.

As long as an animatable property is included in at least one block with
a value that is different then the non-animated attribute value, and
there is a calculable midpoint between those two values, that property
will animate.

Nonanimatable Properties That Aren’t Ignored

Exceptions to the midpoint “rule” include visibility and
animation-timing-function.

visibility is an animatable property, even though there is no midpoint
between visibility: hidden and visibility: visible. When you
animate from hidden to visible, the visibility value jumps from one
value to the next at the keyframe where the change is declared.

While the animation-timing-function is not, in fact, an animatable
property, when included in a keyframe block, the animation timing will switch to
the newly declared value at that point in the animation for the
properties within that keyframe selector block. The change in animation
timing is not animated; it simply switches to the new value for those properties only, and only until the next keyframe. (This will be covered later, in “Changing the Internal Timing of Animations”.)

Scripting @keyframes Animations

There is an API that enables finding,
appending, and deleting keyframe rules. You can change the content of a
keyframe block within an @keyframes animation declaration with
appendRule(n) or deleteRule(n), where n is the full selector of that
keyframe. You can return the contents of a keyframe with findRule(n):

@keyframes W {
  from, to {
    top: 0;
    left: 0;
  }
  25%, 75% {
    top: 100%;
  }
  50% {
    top: 50%;
  }
  to {
    left: 100%;
  }
}

The appendRule(), deleteRule(), and findRule() methods takes the full
keyframe selector as an argument. Revisiting the W animation, to return
the 25% / 75% keyframe, the argument is 25%, 75%:

// Get the selector and content block for a keyframe
var aRule = myAnimation.findRule('25%, 75%').cssText;

// Delete the 50% keyframe
myAnimation.deleteRule('50%');

// Add a 53% keyframe to the end of the animation
myAnimation.appendRule('53% {top: 50%;}');

The statement myAnimation.findRule('25%, 75%').cssText; where
myAnimation is pointing to a keyframe animation, returns the keyframe
that matches 25%, 75%. It would not match anything if we had used
either 25% or 75% only. If pointing to the W animation, this
statement returns 25%, 75% { top: 100%; }.

Similarly, myAnimation.deleteRule('50%') will delete the last 50%
keyframe—so if there are multiple 50% keyframes, the last one listed will be the first to go.
Conversely, myAnimation.appendRule('53% {top: 50%;}') appends a 53% keyframe
after the last keyframe of the @keyframes block.

There are three animation events—animationstart, animationend,
and animationiteration—that occur at the start and end
of an animation, and between the end of an iteration and the start of
a subsequent iteration. Any animation for which a valid keyframe rule is
defined will generate the start and end events, even animations with
empty keyframe rules. The animationiteration event only occurs when an
animation has more than one iteration, as the animationiteration event
does not fire if the animationend event would fire at the same time.

Animating Elements

Once you have created a keyframe animation, you can apply that
animation to elements and/or pseudo-elements.
CSS provides numerous animation properties to
attach a keyframe animation to an element and control its progression. At a minimum, you need to include the name of the animation for the element to
animate, and a duration if you want the animation to actually be visible. (Otherwise, the animation will happen in zero time.)

There are two ways of attaching animation properties to an element: you
can include all the animation properties separately, or you can declare
all the properties in one line using the animation shorthand property (or a combination of shorthand and longhand properties). We are going to first learn all the longhand properties. Later in this
chapter, we’ll condense all the declarations into one line with the
animation shorthand property.

Let’s start with the individual properties.

Naming Animations

The animation-name property takes as its value a
comma-separated list of names of keyframe animations you want to
apply. The names in question here are the unquoted identifiers or quoted strings (or a mixture of both) you
created in your @keyframes rule.

The default value is none, which means there is no animation. The
none value can be used to override any animation applied elsewhere in
the CSS cascade. (This is also the reason you don’t want to name your
animation none, unless you’re a masochist.) To apply an animation,
include the @keyframe identifier, which is the animation name.

Using the change_bgcolor keyframe animation defined in “Omitting from and to Values”:

 div {
     animation-name: change_bgcolor;
 }

This applies the change_bgcolor animation to all div elements.

To apply more than one animation, include more than one comma-separated
@keyframe identifier:

 div {
    animation-name: change_bgcolor, round, W;
 }

If one of the included keyframe identifiers does not exist, the series
of animations will not fail; rather, the failed animation will be
ignored, and the valid animations will be applied. While ignored initially,
the failed animation will be applied if and when that identifier comes into existence as a valid animation. Consider:

 div {
    animation-name: change_bgcolor, spin, round, W;
 }

In this example, assume there is no spin keyframe animation defined. The
spin animation will not be applied, while the change_bgcolor, round,
and W animations will occur. Should a spin keyframe animation come
into existence through scripting, it will be applied at that time.

In order to include more than one animation, we’ve included each @keyframe
animation identifier in our list of comma-separated values on the
animation-name property. If more than one animation is applied to an
element and those animations have repeated properties, the later animations override
the property values in the earlier animations. For example, if more
than two background color changes are applied concurrently in two
different keyframe animations, whichever animation was listed later will
override the background property declarations of animations earlier in the list, but only
if the properties (background colors, in this case) are being animated at the same time. For more
on this, see “Animation, Specificity, and Precedence Order”.

For example, assume the following, and further assume that the animations happen over a period of 10 seconds:

div {animation-name: change_bgcolor, bg-shift;}

@keyframes bg-shift {
    0%, 100% {background-color: blue;}
    35% {background-color: orange;}
    55% {background-color: red;}
    65% {background-color: purple;}
}
@keyframes change_bgcolor {
    0%, 100% {background-color: yellow;}
    45% {background-color: green;}
    55% {background-color: blue;}
}

In this situation, the background will animate from blue to orange to red to purple and then back to blue, thanks to bg-shift. Because it comes last in the list of animations, its keyframes take precedence. Any time there are multiple animations specifying behavior for the same property at the same point in time, the animation which is listed last in the value of animation-name will be in effect.

What’s interesting is what happens if the from (0%) or to (100%) keyframes are omitted from the animation in force. For example, let’s remove the first keyframes defined in bg-shift.

div {animation-name: change_bgcolor, bg-shift;}

@keyframes bg-shift {
    35% {background-color: orange;}
    55% {background-color: red;}
    65% {background-color: purple;}
}
@keyframes change_bgcolor {
    0%, 100% {background-color: yellow;}
    45% {background-color: green;}
    55% {background-color: blue;}
}

Now there are no background colors being defined at the beginning and end of bg-shift. In a situation like this, where a 0% or 100% keyframe is not specified, then the user agent constructs a 0%/100% keyframe using the computed values of the properties being animated. This could mean one of two things: either use the value of the property as defined for the element assuming there are no animations at all, or use the property value from a previous animation in the list given for animation-name.

Older browsers do the former, but the specification is shifting to prefer the latter. As of late 2017, newer browsers will animate from yellow to orange over the first 3.5 seconds of the animation, and from purple to blue over the last 3.5 seconds. Older browsers will start and end with transparent backgrounds.

These are only concerns when two different keyframe blocks are trying to change the same property’s values. In this case, it was background=color. On the other hand, if one keyframe block animates background-color while another animates padding, the two animations will not collide, and both the background color and padding will be animated together.

Simply applying an animation to an element is not enough for the element
to visibly animate, but it will make the animation
occur—just over no time. In such an event, the keyframe properties will all be calculated, and the
animationstart and animationend events will fire. For an element to visibly animate, the animation must last at least some
amount of time. For that we have the animation-duration property.

Defining Animation Lengths

The animation-duration property defines how long a single animation
iteration should take in seconds (s) or milliseconds (ms).

The animation-duration property is used to define the length of
time, in seconds (s) or milliseconds (ms), it should take to complete
one cycle through all the keyframes of the animation.
If you don’t declare animation-duration, the animation will still
be run with a duration of 0s, with animationstart and
animationend still being fired even though the animation, taking 0s,
is imperceptible. Negative time values are not permitted on this property.

When specifying a duration, you must include the second (s) or
millisecond (ms) unit.
If you have more than one animation, you can include a different
animation-duration for each animation by including more than one
comma-separated time duration:

div {
   animation-name: change_bgcolor, round, W;
   animation-duration: 200ms, 100ms, 0.5s;
}

If you supply an invalid value within your comma-separated list of
durations—for example, animation-duration: 200ms, 0, 0.5s—the entire
declaration will fail, and it will behave as if animation-duration: 0s
had been declared. 0 is not a valid time value.

Generally, you will want to include an animation-duration value for
each animation-name provided. If you have only one duration, all the
animations will last the same amount of time. Having fewer
animation-duration values than animation-name values in your
comma-separated property value list will not fail: rather, the values will be repeated as a group. Thus, given the following:

div {
    animation-name: change_bgcolor, spin, round, W;
    animation-duration: 200ms, 5s;
       /* same effect as '200ms, 5s, 200ms, 5s' */
}

the round animation will be run over 200ms, and the W animation over 5s.

If you have a greater number of
animation-duration values than animation-name values, the extra
values will be ignored. If one of the included animations does not
exist, the series of animations and animation durations will not fail:
the failed animation, along with its duration, are ignored:

div {
    animation-name: change_bgcolor, spinner, round, W;
    animation-duration: 200ms, 5s, 100ms, 0.5s;
}

In this example, the duration 5s is associated with spinner.
There is no spinner animation, though, so spinner doesn’t exist, and
the 5s and spinner are both ignored. Should a spinner animation come into existence, it will be applied to the div and last 5 seconds.

Declaring Animation Iterations

Simply including the required animation-name will lead to the animation playing once, and only once. If you want to iterate through the animation more or less than the default one time, use the animation-iteration-count property.

By default, the animation will occur once (because the default value is 1).
If another value is given for animation-iteration-count, and there isn’t a negative
value for the animation-delay property, the animation will repeat the
number of times specified by the value if the property, which can be any
number or the keyword infinite. The following declarations will cause their animations to be repeated 2, 5, and 13 times:

animation-iteration-count: 2;
animation-iteration-count: 5;
animation-iteration-count: 13;

If the value of animation-iteration-count is not an integer, the animation will end partway
through its final cycle. The animation will still run, but will cut off
mid-iteration on the final iteration. For example,
animation-iteration-count: 1.25 will iterate through the animation one and a quarter
times, cutting off 25% of the way through the second iteration. If the value is 0.25
on an 8-second animation, the animation will play about 25% of the way
through, ending after 2 seconds.

Negative numbers are not permitted. If an invalid value is given, the default value of 1 will lead to a default single iteration.

Interestingly, 0 is a valid value for the animation-iteration-count
property. When set to 0, the animation still occurs, but zero times. This
is similar to setting animation-duration: 0s: it will throw both an
animationstart and an animationend event.

If you are attaching more than one animation to an element or
pseudo-element, include a comma-separated list of values for
animation-name, animation-duration, and animation-iteration-count:

.flag {
    animation-name: red, white, blue;
    animation-duration: 2s, 4s, 6s;
    animation-iteration-count: 3, 5;
}

The iteration-count values (and all other animation property values)
will be assigned in the order of the comma-separated animation-name
property value. Extra values are ignored. Missing values cause
the existing values to be repeated, as with animation-iteration-count in the above scenario.

In the preceding example, there are more name values than count values, so the
count values will repeat: red and blue will iterate three times, and
white will iterate five times. There are the same number of name values
as duration values; therefore, the duration values will not repeat. The
red animation lasts two seconds, iterating three times, and therefore will run
for a total of six seconds.
The white animation lasts four seconds, iterating five times,
for a total of 20 seconds. The blue animation is six seconds per
iteration with the repeated three iterations value, animating for a total of
18 seconds.

Invalid values will invalidate the entire declaration, leading to the animations to be played once each.

If we want all three animations to end at the same time, even though
their durations differ, we can control that with
animation-iteration-count:

.flag {
    animation-name: red, white, blue;
    animation-duration: 2s, 4s, 6s;
    animation-iteration-count: 6, 3, 2;
}

In this example, the red, white, and blue animations will last
for a total of 12 seconds each, because the product of the durations and iteration counts in each case totals 12 seconds.

Setting an Animation Direction

With the animation-direction property, you can control whether the
animation progresses from the 0% keyframe to the 100% keyframe, or from
the 100% keyframe to the 0% keyframe. You can control whether all the
iterations progress in the same direction, or set every other animation
cycle to progress in the opposite direction.

The animation-direction property defines the direction of the animation’s
progression through the keyframes. There are four possible
values:

animation-direction: normal

When set to normal (or omitted, which defaults to normal), each
iteration of the animation progresses from the 0% keyframe to the 100%
keyframe.

animation-direction: reverse

The reverse value sets each iteration to play in reverse keyframe
order, always progressing from the 100% keyframe to the 0% keyframe.
Reversing the animation direction also reverses the
animation-timing-function. This property is
described in “Changing the Internal Timing of Animations”.

animation-direction: alternate

The alternate value means the first iteration (and each
subsequent odd-numbered iteration) proceeds from 0% to 100%, and the
second iteration (and each subsequent even-numbered cycle) reverses direction, proceeding from 100% to 0%.

animation-direction: alternate-reverse

The alternate-reverse value is similar to the alternate value,
except it’s the reverse.
The first iteration (and each subsequent odd numbered iteration) proceeds from 100% to 0%, and the second iteration (and each subsequent even-numbered cycle) reverses direction, proceeding from 100% to 0%:

.ball {
    animation-name: bouncing;
    animation-duration: 400ms;
    animation-iteration-count: infinite;
    animation-direction: alternate-reverse;
}
@keyframes bouncing {
    from {
        transform: translateY(500px);
    }
    to {
        transform: translateY(0);
    }
}

In this example, we are bouncing a ball, but we want to start by dropping it, not by throwing it up in the air: we want it to
alternate between going down and up, rather than up and down, so
animation-direction: alternate-reverse is the most appropriate value
for our needs.

This is a rudimentary way of making a ball bounce. When balls are
bouncing, they are moving slowest when they reach their apex and fastest
when they reach their nadir. We included this example here to illustrate
the alternate-reverse animation directions. We’ll revisit the bouncing
animation again later to make it more realistic with the addition of timing (see “Changing the Internal Timing of Animations”).
We’ll also discuss how, when the animation is iterating in the
reverse direction, the animation-timing-function is reversed.

Delaying Animations

The animation-delay property defines how long the browser waits after
the animation is attached to the element before beginning the first
animation iteration.

The animation-delay property sets the time, defined in seconds (s) or
milliseconds (ms), that the animation will wait between when the animation is
attached to the element and when the animation begins executing.

By default, the animation begins iterating as soon as it is applied to the
element, with a 0-second delay. A positive value delays the
start of the animation until the prescribed time listed as the value of
the animation-delay property has elapsed. A negative value causes
the animation to begin immediately, but it will start partway through the animation.

Negative values for
animation-delay can create interesting effects. A negative delay will
execute the animation immediately but will begin animating the element
partway through the attached animation. For example, if
animation-delay: -4s and animation-duration: 10s are set on an
element, the animation will begin immediately but will start
approximately 40% of the way through the first animation, and will end six seconds later.

The word “approximately” was used there because the animation will not necessarily start at precisely the 40%
keyframe block: when the 40% mark of an animation occurs depends on the
value of the animation-timing-function. If
animation-timing-function: linear is set, then it will be 40% through
the animation:

div {
  animation-name: move;
  animation-duration: 10s;
  animation-delay: -4s;
  animation-timing-function: linear;
}

@keyframes move {
  from {
    transform: translateX(0);
  }
  to {
    transform: translateX(1000px);
  }
}

In this linear animation example, we have a 10-second animation
with a delay of –4 seconds. In this case, the animation will start
immediately 40% of the way through the animation, with the div translated
400 pixels to the right of its original position, and last only six seconds.

If an animation is set to occur 10 times, with a delay of -600 milliseconds and
an animation duration of 200 milliseconds, the element will start animating right away, at the beginning of the fourth iteration:

.ball {
  animation-name: bounce;
  animation-duration: 200ms;
  animation-delay: -600ms;
  animation-iteration-count: 10;
  animation-timing-function: ease-in;
  animation-direction: alternate;
}
@keyframes bounce {
  from {
    transform: translateY(0);
  }
  to {
    transform: translateY(500px);
  }
}

Instead of animating for 2,000 milliseconds (200 ms × 10 = 2,000 ms, or 2
seconds), starting in the normal direction, the ball will animate for
1,400 milliseconds (or 1.4 seconds) with the animation starting immediately—but at the start of the fourth iteration, and in the reverse direction.

It starts out in reverse because
animation-direction is set to alternate, meaning every even iteration
iterates in the reverse direction from the 100% keyframe to the 0% keyframe.
The fourth iteration, which is an even-numbered iteration, is the first
visible iteration.

The animation will throw the animationstart event immediately. The
animationend event will occur at the 1,400-millisecond mark. The ball will be
tossed up, rather than bounced, throwing 6 animationiteration events,
after 200, 400, 600, 800, 1,000, and 1,200 milliseconds. While the iteration count was
set to 10, we only get 6 animationiteration events because we are only
getting 7 iterations; 3 iterations didn’t occur because of the negative animation-delay, and the last iteration concluded at the same time as the
animationend event. Remember, when an animationiteration event
would occur at the same time as an animationend event, the
animationiteration event does not occur.

Let’s take a deeper look at animation events before continuing.

Animation Events

There are three different types of animation events:
animationstart, animationiteration, and animationend.
Each event has three read-only properties:
animationName, elapsedTime, and pseudoElement, unprefixed in all
browsers.

The animationstart event fires at the start of the animation: after
the animation-delay (if present) has expired, or immediately if there
is no delay set. If a negative animation-delay value is present, the
animationstart will fire immediately, with an elapsedTime equal to
the absolute value of the delay in supporting browsers. In browsers
where prefixing is still necessary, the elapsedTime is 0:

.noAnimationEnd {
    animation-name: myAnimation;
    animation-duration: 1s;
    animation-iteration-count: infinite;
}
.startAndEndSimultaneously {
    animation-name: myAnimation;
    animation-duration: 0s;
    animation-iteration-count: infinite;
}

The animationend event fires when the animation finishes. If the
animation-iteration-count is set to infinite, then as long as the
animation-duration is set to a time greater than 0, the animationend event will
never fire. If the animation-duration is set or defaults to 0 seconds, even
when the iteration count is infinite, animationstart and
animationend will occur virtually simultaneously, and in that order.

The animationiteration event fires between iterations. The animationend event fires at the conclusion of iterations that do not occur at the same time as the conclusion of the animation itself; thus, the animationiteration and animationend events do not fire simultaneously:

.noAnimationIteration {
    animation-name: myAnimation;
    animation-duration: 1s;
    animation-iteration-count: 1;
}

In the .noAnimationIteration example, with the
animation-iteration-count set to a single occurrence, the animation
ends at the conclusion of the first and only iteration. Whenever the
animationiteration event would occur at the same time as an animationend event, the animationend event occurs, but
the animationiteration event does not. The animationiteration does
not fire unless an animation cycle ends and another begins.

When the animation-iteration-count property is omitted, or when its
value is 1 or less, no animationiteration event will be fired. As long
as an iteration finishes (even if it’s a partial iteration) and another
iteration begins, if the duration of that subsequent iteration is greater than 0s, an
animationiteration event will be fired:

.noAnimationIteration {
    animation-name: myAnimation;
    animation-duration: 1s;
    animation-iteration-count: 4;
    animation-delay: -3s;
}

When an animation iterates through fewer cycles than listed in the
animation-iteration-count because of a negative animation-delay, there
are no animationiteration events for the cycles that didn’t occur. In the
preceding example code, there are no animationiteration events, as the first
three cycles do not occur (due to the -3s animation-delay), and the last
cycle finishes at the same time the animation ends.

In that example, the elapsedTime on the animationstart event is
3, as it is equal to the absolute value of the delay.

Animation chaining

You can use animation-delay to chain animations together so the next
animation starts immediately after the conclusion of the preceding
animation:

.rainbow {
    animation-name: red, orange, yellow, blue, green;
    animation-duration: 1s, 3s, 5s, 7s, 11s;
    animation-delay: 3s, 4s, 7s, 12s, 19s;
}

In this example, the red animation starts after a three-second delay and lasts
one second, meaning the animationend event occurs at the four-second mark. This
example starts each subsequent animation at the conclusion of the
previous animation. This is known as CSS animation chaining.

By including a four-second delay on the second animation, the orange animation
will begin interpolating the @keyframe property values at the four-second mark,
starting the orange animation immediately at the conclusion of the
red animation. The orange animation concludes at the seven-second mark—it
lasts three seconds, starting after a four-second delay—which is the delay set on the
third, or yellow, animation, making the yellow animation begin
immediately after the orange animation ends.

This is an example of chaining animations on a single element. You can
also use the animation-delay property to chain the animations for
different elements:

li:first-of-type {
    animation-name: red;
    animation-duration: 1s;
    animation-delay: 3s;
}
li:nth-of-type(2) {
    animation-name: orange;
    animation-duration: 3s;
    animation-delay: 4s;
}
li:nth-of-type(3)  {
    animation-name: yellow;
    animation-duration: 5s;
    animation-delay: 7s;
}
li:nth-of-type(4) {
    animation-name: green;
    animation-duration: 7s;
    animation-delay: 12s;
}
li:nth-of-type(5) {
    animation-name: blue;
    animation-duration: 11s;
    animation-delay: 19s;
}

If you want a group of list items to animate in order, appearing as if
the animations were chained in sequence, the animation-delay of each
list item should be the combined time of the animation-duration and
animation-delay of the previous animation.

While you can use JavaScript and the animationEnd event from one animation to determine when to attach a subsequent animation, which we discuss below, the animation-delay property is an appropriate method of using CSS
animation properties to chain animations. There is one caveat: animations are the lowest priority on the UI thread. Therefore, if you
have a script running that is occupying the user interface (or UI)
thread, depending on the browser and which properties are being animated
and what property values are set on the element, the browser may let the delays expire while waiting
until the UI thread is available before starting more animations.

If you are able to rely on JavaScript, another way of chaining
animations is listening for animationend events to start subsequent
animations:

<script>
  document.querySelectorAll('li')[0].addEventListener( 'animationend',
    function(e) {
        document.querySelectorAll('li')[1].style.animationName = 'orange';
    },
    false );

  document.querySelectorAll('li')[1].addEventListener( 'animationend',
    function(e) {
        document.querySelectorAll('li')[2].style.animationName = 'yellow';
    },
    false );

  document.querySelectorAll('li')[2].addEventListener( 'animationend',
    function(e) {
        document.querySelectorAll('li')[3].style.animationName = 'green';
    },
    false );

  document.querySelectorAll('li')[3].addEventListener( 'animationend',
    function(e) {
        document.querySelectorAll('li')[4].style.animationName = 'blue';
    },
    false );
</script>

<style>
  li:first-of-type {
    animation-name: red;
    animation-duration: 1s;
  }
  li:nth-of-type(2) {
    animation-duration: 3s;
  }
  li:nth-of-type(3)  {
    animation-duration: 5s;
  }
  li:nth-of-type(4) {
    animation-duration: 7s;
  }
  li:nth-of-type(5)  {
    animation-duration: 11s;
  }
</style>

In this example, there is an event handler on each of the first four list
items, listening for that list item’s animationend event. When the
animationend event occurs, the event listeners add an animation-name
to the subsequent list item.

As you can see in the styles, this animation chaining method doesn’t employ animation-delay at all. Instead, the JavaScript event listeners
attach animations to each element by setting the animation-name
property when the animationend event is thrown.

You’ll also note that the animation-name was only included
for the first list item. The other list items only have an animation-duration with no animation-name, and therefore no attached
animations. Adding animation-name is what attaches and starts the
animation. To start or restart an animation, the animation name
must be removed and then added back—at which point all the
animation properties take effect, including animation-delay.

Instead of writing:

<script>
  document.querySelectorAll('li')[2].addEventListener( 'animationend',
    function(e) {
        document.querySelectorAll('li')[3].style.animationName = 'green';
    },
    false );

  document.querySelectorAll('li')[3].addEventListener( 'animationend',
    function(e) {
        document.querySelectorAll('li')[4].style.animationName = 'blue';
    },
    false );
</script

<style>
  li:nth-of-type(4) {
    animation-duration: 7s;
  }
  li:nth-of-type(5)  {
    animation-duration: 11s;
  }
</style>

we could have written:

<script>
  document.querySelectorAll('li')[2].addEventListener( 'animationend',
    function(e) {
        document.querySelectorAll('li')[3].style.animationName = 'green';
        document.querySelectorAll('li')[4].style.animationName = 'blue';
    },
  false );
</script>

<style>
  li:nth-of-type(4) {
    animation-duration: 7s;
  }
  li:nth-of-type(5)  {
    animation-delay: 7s;
    animation-duration: 11s;
  }
</style>

When the blue animation name is added to the fifth list item at the same time we added green, the delay on the fifth
element takes effect at that point in time and starts expiring.

Note

While changing the values of animation properties (other than name) on the element during an animation has no effect on the animation,
removing or adding an animation-name does have an impact. You can’t
change the animation duration from 100ms to 400ms in the middle of an
animation. You can’t switch the delay from -200ms to 5s once the delay
has already been applied. You can, however, stop and start the animation
by removing it and reapplying it. In this JavaScript example, we
started the animations by applying them to the elements.

In addition, setting display: none on an element terminates any
animation.
Updating the display back to a visible value restarts the
animation from the beginning. If there is a positive value for
animation-delay, the delay will have to expire before the
animationstart event happens and any animations occur. If the delay is
negative, the animation will start midway through an iteration, exactly as
it would have if the animation had been applied any other way.

Animation iteration delay

While there is no such property as an animation-iteration-delay, you
can employ the animation-delay property, incorporate delays within
your keyframe declaration, or use JavaScript to fake it. The best method
for faking it depends on the number of iterations, performance, and
whether the delays are all equal in length.

What is an animation iteration delay? Sometimes you want an animation to
occur multiple times, but want to wait a specific amount of time between
each iteration.

Let’s say you want your element to grow three times, but want to wait four
seconds between each one-second iteration. You can include the delay within your
keyframe definition and iterate through it three times:

.animate3times {
    background-color: red;
    animation: color_and_scale_after_delay;
    animation-iteration-count: 3;
    animation-duration: 5s;
}

@keyframes color_and_scale_after_delay {
    80% {
        transform: scale(1);
        background-color: red;
    }
    80.1% {
        background-color: green;
        transform: scale(0.5);
    }
    100% {
        background-color: yellow;
        transform: scale(1.5);
    }
}

Note the first keyframe selector is at the 80% mark and matches the
default state. This will animate your element three times: it stays in the default state for 80% of the five-second animation (or four seconds) and then moves from green to yellow and small to big over the last one second of the animation before iterating again, stopping after three iterations.

This method works for any number of iterations of the animation.
Unfortunately, it is only a good solution if the delay between
each iteration is identical and you don’t want to reuse the animation
with any other timing, such as a delay of six seconds. If you want
to change the delay between each iteration while not changing the
duration of the change in size and color, you have to write a new
@keyframes definition.

To enable different iteration delays between animations, we could create
a single animation and bake in the effect of three different delays:

.animate3times {
    background-color: red;
    animation: color_and_scale_3_times;
    animation-iteration-count: 1;
    animation-duration: 15s;
}

@keyframes color_and_scale_3_times {
  0%, 13.32%, 20.01%, 40%, 46.67%, 93.32% {
        transform: scale(1);
        background-color: red;
  }
    13.33%, 40.01%, 93.33% {
        background-color: green;
        transform: scale(0.5);
  }
    20%, 46.66%, 100% {
        background-color: yellow;
        transform: scale(1.5);
  }
}

This method may be more difficult to code and maintain. It works for a
single cycle of the animation. To change the number of animations or the
iteration delay durations, another @keyframes declaration would be
required. This example is even less robust than the previous one, but it
does allow for different between-iteration delays.

There’s a solution that works in most browsers that is now
specifically allowed in the animation specification: declare an animation multiple times, each
with a different animation-delay value:

.animate3times {
  animation: color_and_scale, color_and_scale, color_and_scale;
  animation-delay: 0, 4s, 10s;
  animation-duration: 1s;
}

@keyframes color_and_scale {
    0% {
        background-color: green;
        transform: scale(0.5);
    }
    100% {
        background-color: yellow;
        transform: scale(1.5);
    }
}

Here, we’ve attached the animation three times, each with a different delay.
In this case, each animation iteration concludes before the next one
proceeds.

If animations overlap while they’re concurrently animating, the values
will be the values from the last declared animation. As is true whenever
there are multiple animations changing an element’s property at the
same time, the animation that occurs last in the sequence of animation
names will override any animations occurring before it in the list of
names. In declaring three color_and_scale animations but at different
intervals, the value of the property of the last iteration of the
color_and_scale animation will override the values of the previous
ones that haven’t yet concluded.

The safest, most robust and most cross-browser-friendly method of faking
an animation-iteration-delay property is to use animation events. On
animationend, detach the animation from the element, then reattach it after
the iteration delay. If all the iteration delays are the same, you can
use setInterval; if they vary, use setTimeout:

var iteration = 0;
var el = document.getElementById('myElement');

el.addEventListener('animationend', function(e) {
  var time = ++iteration * 1000;

  el.classList.remove('animationClass');

  setTimeout(function() {
    el.classList.add('animationClass');
  }, time);

});

This example animates
myElement infinitely, adding an additional second between each iteration
of the animation.

Changing the Internal Timing of Animations

All right! The scripting was fun, but let’s get back to straight CSS and talk about timing functions.
Similar to the transition-timing-function property, the
animation-timing-function property describes how the animation will
progress over one cycle of its duration, or iteration.

Other than the step
timing functions, described in “Step timing functions”, the timing functions are all Bézier
curves. Just like the transition-timing-function, the CSS specification provides for five predefined Bézier curve
keywords, as shown in Figure 18-1 and Table 18-1.

Figure 18-1. Cubic Bézier named functions
Table 18-1. Bézier curve keywords
Timing function Cubic Bézier value

ease

cubic-bezier(0.25, 0.1, 0.25, 1)

linear

cubic-bezier(0, 0, 1, 1)

ease-in

cubic-bezier(0.42, 0, 1, 1)

ease-out

cubic-bezier(0, 0, 0.58, 1)

ease-in-out

cubic-bezier(0.42, 0, 0.58, 1)

A handy tool to visualize Bézier curves and to create your own is Lea
Verou’s cubic Bézier visualizer at cubic-bezier.com.

The default ease has a slow start, then speeds up, and ends slowly.
This function is similar to ease-in-out, which has a greater acceleration at the beginning.
linear, as the name describes, creates an animation that animates at a constant speed.

ease-in creates an animation that is slow to start, gains speed, and then stops abruptly.
The opposite ease-out timing function starts at full speed, then slows
progressively as it reaches the conclusion of the animation iteration.

If none of these work for you, you can create your own Bézier curve
timing function by passing four values, such as:

animation-timing-function: cubic-bezier(0.2, 0.4, 0.6, 0.8);

Bézier curves are mathematically defined parametric curves used in two-dimensional graphic applications. See Appendix A for examples of curves you can define yourself in CSS.

The Bézier curve takes four values, defining the originating position of
the two handles. In CSS, the anchors are at 0, 0 and 1, 1. The first two values define the x and y of the first
point or handle of the curve, and the last two are the x and y of the
second handle of the curve. The x values must be between 0 and 1, or the
Bézier curve is invalid. The y coordinate is not constrained. When creating your own Bézier curve, remember:
the steeper the curve, the faster the motion. The flatter the curve, the
slower the motion.1

While the x values must be between 0 and 1, by using values for y that
are greater than 1 or less than 0, you can create a bouncing effect,
making the animation bounce up and down between values, rather than
going consistently in a single direction. Consider the following timing function, whose rather outlandish Bézier curve is (partly) illustrated in Figure 18-2:

.snake {
  animation-name: shrink;
  animation-duration: 10s;
  animation-timing-function: cubic-bezier(0, 4, 1, -4);
  animation-fill-mode: both;
}

@keyframes shrink {
  0% {
    width: 500px;
  }
  100% {
    width: 100px;
  }
}

Figure 18-2. An outlandish Bézier curve

This animation-timing-function curve makes the animated property’s values go
outside the boundaries of the values set in the 0% and 100%
keyframes. In this example, we are shrinking an element from 500px to
100px. However, because of the cubic-bezier values, the element
we’re shrinking will actually grow to be wider than the 500px width
defined in the 0% keyframe and narrower than the 100px width
defined in the 100% keyframe, as shown in Figure 18-3.

Figure 18-3. Effect of outlandish Bézier curve

In this scenario, the element starts with a width of 500px, defined in the 0%
keyframe. It then quickly shrinks down to a width of about 40px, which
is narrower than width: 100px defined in the 100%
keyframe. From there, it slowly expands to about 750px wide, which is larger
than the original width of 500px. It then quickly shrinks back down to
width: 100px, ending the animation iteration.

You may have realized that the curve created by our animation is the same
curve as the Bézier curve. Just as the S-curve goes outside the normal bounding box, the width
of the animated element goes narrower than the smaller width we set of 100px,
and wider than the larger width we set of 500px.

The Bézier curve has the appearance of a snake because one y coordinate is positive, and the other negative. If
both are positive values greater than 1 or both are negative values less than -1,
the Bézier curve is arc-shaped, going above or below one of the values
set, but not bouncing out of bounds on both ends like the S-curve.

Any timing function declared with animation-timing-function sets the
timing for the normal animation direction, when the animation is
progressing from the 0% keyframe to the 100% keyframe.
When the animation is running
in the reverse direction, from the 100% keyframe to the 0% keyframe, the
animation timing function is reversed.

Remember the bouncing-ball example? The bouncing wasn’t very realistic, because the original example defaulted to ease for its timing function.
With animation-timing-function, we can apply ease-in to the animation so that when the ball is dropping, it gets faster as it nears
its nadir at the 100% keyframe.
When it is bouncing upward, it animates in the reverse
direction, from 100% to 0%, so the animating-timing-function is
reversed as well—in this case to ease-out—slowing down as it reaches the apex:

.ball {
  animation-name: bounce;
  animation-duration: 1s;
  animation-iteration-count: infinite;
  animation-timing-function: ease-in;
  animation-direction: alternate;
}

@keyframes bounce {
  0% {
    transform: translateY(0);
  }
  100% {
    transform: translateY(500px);
  }
}

Step timing functions

The step timing functions, step-start, step-end, and steps(),
aren’t Bézier curves. They’re not really curves at all. Rather, they’re tweening definitions. The steps() function is most useful when it comes to character or sprite animation.

The steps() timing function divides the animation into a series of
equal-length steps. steps() takes two parameters: the number of steps,
and the change point (more on that in a moment).

The number of steps is the first parameter; its value must be a
positive integer. The animation length will be divided equally into the number
of steps provided. For example, if the animation duration is 1 second
and the number of steps is 5, the animation will be divided into five
200-millisecond steps, with the element being redrawn to the page five times, at
200-millisecond intervals, moving 20% through the animation at each interval.

To understand how this works, think of a flip book.
Each page in a flip book contains a single drawing or picture that changes
slightly from one page to the next, like one frame from a movie reel stamped onto each page. When the pages of a flip book are
rapidly flipped through (hence the name), the pictures appear as an
animated motion. You can create similar animations with CSS using an
image sprite, the background-position property, and the steps()
timing function.

Figure 18-4 shows an image sprite containing several images that change just slightly,
like the drawings on the individual pages of a flip book.

Figure 18-4. Sprite of dancing

We put all of our slightly differing images into a single image called a sprite. Each image in our sprite is a frame in the
single animated image we’re creating.

We then create a container element that is the size of a single image of our
sprite, and attach the sprite as the container element’s background image. We then animate the background-position, using the steps()
timing function so we only see a single instance of the changing image
of our sprite at a time. The number of steps in our steps() timing
function is the number of occurrences of the image in our sprite. The
number of steps defines how many stops our background image makes to
complete a single animation.

The sprite in Figure 18-4 has 22 images, each 56 × 100 pixels. The total size of our
sprite is 1,232 × 100 pixels. We set our container to the individual image
size: 56 × 100 pixels. We set our sprite as our background image: the
initial or default value of background-position is top left, which
is the same as 0 0. Our image will appear at 0 0, which is a good
default: older browsers that don’t support CSS animation will simply
display the first image from our sprite:

.dancer {
  height: 100px;
  width: 56px;
  background-image: url(../images/dancer.png);
  ....
}

The trick is to use steps() to change the background-position value
so that each frame is a view of a separate image within the sprite.
Instead of sliding in the background image from the left, the steps()
timing function will pop in the background image in the number of steps
we declared.

So we create an animation that simple changes the left-right value
of the background-position. The image is 1,232 pixels wide, so we move
the background image from 0 0, which is the left top, to
0 -1232px, putting the sprite fully outside of our 56 × 100 pixel
div viewport.

The values of -1232px 0 will move the image completely to the left,
outside of our containing block viewport. It will no longer show up as a
background image in our 100 × 56 pixel div at the 100% mark unless
background-repeat is set to repeat along the x-axis. We don’t want that to happen!

This is what we want:

@keyframes dance_in_place {
  from {
      background-position: 0 0;
  }
  to {
      background-position: -1232px 0;
  }
}

.dancer {
  ....
  background-image: url(../images/dancer.png);
  animation-name: dance_in_place;
  animation-duration: 4s;
  animation-timing-function: steps(22, end);
  animation-iteration-count: infinite;
}

What may have seemed like a
complex animation is very simple: just like a flip book, we see one
frame of the sprite at a time. Our keyframe animation simply moves the
background.

So that covers the first parameter, the number of steps.
The second parameter takes one of two values: either start or
end. What this specifies whether the change for the first step’s interval takes place at the beginning of that interval, or at the end of the interval.
With the default value, end, the change take place at the end of the first step. In other words, given 200 ms step lengths, the first change in the animation will not occur until 200 ms into the animation’s overall duration. With start, the first change will take place at the beginning of the first step’s interval; that is to say, the instant the animation begins. Figure 18-5 provides a timeline diagram of how the two values work, based on the following styles:

@keyframes grayfade {
    from {background-color: #BBB;}
    to {background-color: #333;}
}

.quickfader {animation: grayfade 1s steps(5,start) forwards;}
.slowfader  {animation: grayfade 1s steps(5,end) forwards;}

Figure 18-5. Visualizing start and end change points

The boxes embedded into each timeline represent the background color during that step interval. Notice that in the end timeline, the first interval is the same as the background before the animation started. This is because the animation waits until the end of the first frame to make the color change for the first step (the color between “Step 1” and “Step 2”).

In the start timeline, on the other hand, the first interval makes that color change at the start of the interval, instantly switching from the starting background color to the color between “Step 1” and “Step 2”. This is sort of like jumping ahead one interval, an impression reinforced by the fact that the background color in “Step 2” of the end timeline is the same as that in “Step 1” of the start timeline.

A similar effect can be seen at the end of each animation, where the background in the fifth step of the start timeline is the same as the ending background color. In the end timeline, it’s the color at the point between “Step 4” and “Step 5”, and doesn’t switch to the ending background color until the end of “Step 5,” when the animation is finished.

The change parameter can be hard to keep straight. If it helps, think of it
this way: in a normal animation direction, the start value “skips” the 0% keyframe, because it makes the first change as soon as the animation starts, and the end value
“skips” the 100% keyframe.

Note

Preserving the ending background color in this case, rather than having it reset to the starting color after the animation finishes, required the presences of the forwards keyword. We’ll cover that in “Animation Fill Modes”, later in the chapter.

The step-start value is equal to steps(1, start), with only a single
step displaying the 100% keyframe. The step-end value is equal to
steps(1, end), which displays only the 0% keyframe.

Adding a second animation

Let’s go back to the sprite animation, which shows our tiny dancer dancing in place. Most dancers move around when they
dance. We can add a little left-and-right and back-and-forth motion by
adding a second animation:

@keyframes move_around {
  0%, 100% {
    transform: translate(0, -40px) scale(0.9);
  }
  25%  {
    transform: translate(40px, 0)  scale(1);
  }
  50%  {
    transform: translate(0, 40px)  scale(1.1);
  }
  75%  {
    transform: translate(-40px, 0) scale(1);
  }
}

Here, we create a second keyframe animation called move_around and attach
it to our dancer element as a second animation with comma-separated
animation property declarations:

.dancer {
    ....
    background-image: url(../images/dancer.png);
    animation-name: dance_in_place, move_around;
    animation-duration: 4s, 16s;
    animation-timing-function: steps(22, end), steps(5, end);
    animation-iteration-count: infinite;
}

Note that each animation property has two comma-separated values except
animation-iteration-count. If you recall, if an animation property
doesn’t have enough comma-separated values to match the number of
animations declared by the animation-name property, the values present
will be repeated until there are enough. We want both animations to
continue indefinitely. As the value of infinite is for all the
attached animations, we only need a single value for that property. The
browser will repeat the list of animation-iteration-count values—in
this case, just the single value of infinite—until it has matched
an animation-iteration-count value for each animation declared.

Animating the timing function

The animation-timing-function is not an animatable property, but it
can be included in keyframes to alter the current timing of the
animation.

Unlike animatable
properties, the animation-timing-function values aren’t interpolated
over time. When included in a keyframe within the @keyframes definition,
the timing function for the properties declared within that same keyframe will change to the new animation-timing-function
value when that keyframe is reached, as shown in Figure 18-6:

@keyframes width {
  0% {
    width: 200px;
    animation-timing-function: linear;
  }
  50% {
    width: 350px;
    animation-timing-function: ease-in;
  }
  100% {
    width: 500px;
  }
}

Figure 18-6. Changing the animation timing function mid-animation

In the preceding example, as shown in Figure 18-6, halfway through the animation, we
switch from a linear animation progression for the width property to
one that eases in. The ease-in timing starts from the keyframe in which the timing function changes.

Specifying the animation-timing-function within the to or 100%
keyframe will have no effect on the animation. When included in any other keyframe, the animation will follow the
animation-timing-function specified in that keyframe definition until it reaches the next keyframe,
overriding the element’s default or declared animation-timing-function.

If the animation-timing-function property is included in a keyframe,
only the properties also included in that keyframe block will have their
timing function impacted. The new timing function will be in play on that property until the next keyframe containing that property is reached, at which point it will change to the timing function declared within that block, or revert back to the original timing function assigned to that element. If we take our W animation as an example:

@keyframes W {
    from {
      left: 0;
      top: 0;
    }
    25%, 75% {
      top: 100%;
    }
    50% {
      top: 50%;
    }
    to {
      left: 100%;
      top: 0;
    }
}

This follows the idea that conceptually, when an animation is set on an
element or pseudo-element, it is as if a set of keyframes is created for
each property that is present in any of the keyframes, as if an
animation is run independently for each property that is being animated.
It’s as if the W animation were made up of two animations that run
simultaneously—W_part1 and W_part2:

@keyframes W_part1 {
    from, to {
      top: 0;
    }
    25%, 75% {
      top: 100%;
    }
    50% {
      top: 50%;
    }
}

@keyframes W_part2 {
    from {
      left: 0;
    }
    to {
      left: 100%;
    }
}

The animation-timing-function that is set on any of the keyframes is
added to the progression of only the properties that are defined at that
keyframe:

@keyframes W {
    from {
      left: 0;
      top: 0;
    }
    25%, 75% {
      top: 100%;
    }
    50% {
      animation-timing-function: ease-in;
      top: 50%;
    }
    to {
      left: 100%;
      top: 0;
    }
}

The preceding code will change the
animation-timing-function to ease-in for the top property only, not the left property,
impacting only the W_part1 section of our W animation, and only from the middle of the animation to the 75% mark.

However, with the following animation, the animation-timing-function will have no effect, because it’s been placed in a
keyframe block that has no property/value declarations:

@keyframes W {
    from {
      left: 0;
      top: 0;
    }
    25%, 75% {
      top: 100%;
    }
    50% {
      animation-timing-function: ease-in;
    }
    50% {
      top: 50%;
    }
    to {
      left: 100%;
      top: 0;
    }
}

How is it useful to change the timing function mid-animation? In the
bounce animation, we had a frictionless environment: the ball
bounced forever, never losing momentum. The ball sped up as it
dropped and slowed as it rose, because the timing function was
inverted from ease-in to ease-out by default as the animation
proceeded from the normal to reverse direction every other iteration.

In reality, friction exists; momentum is lost. Balls will not continue
to bounce indefinitely. If we want our bouncing ball to look natural, we
have to make it bounce less high as it loses energy with each impact. To
do this, we need a single animation that bounces multiple times, losing
momentum on each bounce, while switching between ease-in and ease-out at each
apex and nadir:

@keyframes bounce {
  0% {
    transform: translateY(0);
    animation-timing-function: ease-in;
  }
  30% {
    transform: translateY(100px);
    animation-timing-function: ease-in;
  }
  58% {
    transform: translateY(200px);
    animation-timing-function: ease-in;
  }
  80% {
    transform: translateY(300px);
    animation-timing-function: ease-in;
  }
  95% {
    transform: translateY(360px);
    animation-timing-function: ease-in;
  }
  15%, 45%, 71%, 89%, 100% {
    transform: translateY(380px);
    animation-timing-function: ease-out;
  }
}

This animation loses height after a few bounces, eventually stopping.

Since this new animation uses a single iteration, we can’t rely on the
animation-direction to change our timing function. We need to
ensure that while each bounce causes the ball to lose momentum, it still
speeds up with gravity and slows down as it reaches its apex. Because
we will have only a single iteration, we control the timing by including
animation-timing-function within our keyframes. At every apex, we
switch to ease-in, and at every nadir, or bounce, we switch to
ease-out.

Setting the Animation Play State

If you need to pause and resume animations,
the animation-play-state property defines whether the animation is
running or paused.

When set to the default running, the animation proceeds as normal. If
set to paused, the animation will be paused. When paused, the
animation is still applied to the element, halted at the progress it had
made before being paused. If stopped mid iteration, the properties that were in the process of animating stay at their mid-iteration values. When set back to running or returned to the
default of running, it restarts from where it left off, as if the
“clock” that controls the animation had stopped and started again.

If the property is set to animation-play-state: paused during the
delay phase of the animation, the delay clock is also paused and resumes
expiring as soon as animation-play-state is set back to running.

Animation Fill Modes

The animation-fill-mode property enables us to define whether or not
an element’s property values continue to be applied by the animation outside of the animation execution.

This property is useful because by default, the changes in an animation only apply during the animation itself. Once the animation is done, the values will all revert to their pre-animation values. Thus, if you animate the background from red to blue, the background will (by default) revert to red once the animation finishes.

Similarly, an animation will not affect the property values of the
element immediately if there is a positive animation-delay applied.
Rather, animation property values are applied when the animation-delay
expires, when the animationstart event is fired.

With animation-fill-mode, we can define how the animation impacts the
element on which it is set before the animationstart and after the
animationend events are fired. Property values
set in the 0% keyframe can be applied to the element during the expiration
of any animation delay, and property values can continue to persist after the
animationend event is fired.

The default value for animation-fill-mode is none, which means the animation has no effect
when it is not executing: the animation’s 0% keyframe (or the 100% keyframe in a reverse animation) block property
values are not applied to the animated element until the
animation-delay has expired, when the animationstart event is fired.

When the value is set to backwards, the property values from the 0%
or from keyframe (if there is one) will be applied to the element as
soon as the animation is applied to the element. The 0% keyframe
property values are applied immediately (or 100% keyframe, if the value of the animation-direction property is
reversed or reversed-alternate), without waiting for the animation-delay time to expire.

The value of forwards means when the animation is done executing—that is, has concluded the last part of the last iteration as defined by the
animation-iteration-count value, and the animationend event has fired—it continues to apply the values of
the properties as they were when the animationend event
occurred. If the iteration-count has an integer value, this will be
either the 100% keyframe, or, if the last iteration was in the reverse
direction, the 0% keyframe.

The value of both applies both the backwards effect of applying the
property values as soon as the animation is attached to the element, and the
forwards value of persisting the property values past the
animationend event.

If the animation-iteration-count is a float value, and not an integer,
the last iteration will not end on the 0% or 100% keyframe: the
animation will instead end its execution partway through an animation cycle. If
the animation-fill-mode is set forwards or both, the element
maintains the property values it had when the animationend event
occurred. For example, if the animation-iteration-count is 6.5, and
the animation-timing-function is linear, the animationend event fires
and the values of the properties at the 50% mark (whether or not a 50%
keyframe is explicitly declared) will stick, as if the
animation-play-state had been set to pause at that point.

For example, if we take the following code:

@keyframes move_me {
  0% {
    transform: translateX(0);
  }
  100% {
    transform: translateX(1000px);
  }
}

.moved {
  transform: translateX(0);
  animation-name: move_me;
  animation-duration: 10s;
  animation-timing-function: linear;
  animation-iteration-count: 0.6;
  animation-fill-mode: forwards;
}

The animation will only go through 0.6 iterations. Being a linear 10-second
animation, it will stop at the 60% mark, 6 seconds into the animation,
when the element is translated 600 pixels to the right. With
animation-fill-mode set to forwards or both, the animation will
stop animating when it is translated 600 pixels to the right, holding the
moved element 600 pixels to the right of its original position. This will keep it translated indefinitely, or at least until the animation is detached from the element. Without the animation-fill-mode: forwards, the element with class moved will pop back to it’s original transform: translateX(0), as defined in the moved selector code block.

Warning

In Safari 9 and earlier, forwards and both will set the values from
the 100% keyframe onto the element, no matter the direction of the last
iteration or whether the animation otherwise ended on the 100% keyframe
or elsewhere in the animation. In the preceding example, in Safari 9, the
.moved element jumps from being translated by 400 pixels to the right
to be 1,000 pixels to the right of where it normally would have been, and stays
there. In Safari 9 and earlier, it doesn’t matter whether the last iteration was
normal or reverse, or whether the animation ended 25% or 75% of the
way through an animation cycle; animation-fill-mode: forwards causes
the animation to jump to the 100% frame and stay there. This follows an
older version of the specification, but we expect it will be updated to
match the updated specification and all other evergreen browsers.

Bringing It All Together

The animation shorthand property allows you to use one line, instead of
eight, to define all the animation parameters for an element. The
animation property value is a list of space-separated values for the
various longhand animation properties. If you are setting multiple
animations on an element or pseudo-element, you can use a comma-separated list of animations.

The animation shorthand takes as its value all the other preceding animation
properties, including animation-duration,
animation-timing-function, animation-delay,
animation-iteration-count, animation-direction,
animation-fill-mode, animation-play-state, and animation-name. For example, the following two rules are precisely equivalent:

#animated {
  animation: 200ms ease-in 50ms 1 normal running forwards slidedown;
}
#animated {
  animation-name: slidedown;
  animation-duration: 200ms;
  animation-timing-function: ease-in;
  animation-delay: 50ms;
  animation-iteration-count: 1;
  animation-fill-mode: forwards;
  animation-direction: normal;
  animation-play-state: running;
}

We didn’t have to declare all of the values in the animation shorthand;
any values that aren’t declared are set to the default or initial
values. The first shorthand line was long and three of the
properties were set to default, so were not necessary.

It’s important to remember that if you don’t declare all eight values in
your shorthand declaration, the ones you don’t declare will get the
initial value for that property. The initial or default values are:

animation-name: none;
animation-duration: 0s;
animation-timing-function: ease;
animation-delay: 0;
animation-iteration-count: 1;
animation-fill-mode: none;
animation-direction: normal;
animation-play-state: running;

The order of the shorthand is important in two very specific ways. First, there
are two time properties permitted, for <animation-duration> and <animation-delay>. When two are listed, the first is always the duration. The second, if present, is interpreted as the delay.

Second, the placement of the animation-name is also important. If you use
an animation property value as your animation identifier—which you
shouldn’t, but say that you do—then
the animation-name should be placed as the last property
value in the animation shorthand. The first occurrence of a keyword
that is a valid value for any of the other animation properties, such as
ease or running, is assumed to be part of the shorthand of the
animation property the keyword is associated with, rather than the
animation-name. Note that none is basically the only word that is
not a valid animation name:

#failedAnimation {
    animation: paused 2s;
}

This is the equivalent to:

 #failedAnimation {
    animation-name: none;
    animation-duration: 2s;
    animation-delay: 0;
    animation-timing-function: ease;
    animation-iteration-count: 1;
    animation-fill-mode: none;
    animation-direction: normal;
    animation-play-state: paused;
}

paused is a valid animation name. While it may seem that the animation
named paused with a duration of 2s is being attached to the element,
that is not what is happening in the above shorthand. Because words within the shorthand
animation are first checked against possible valid values of all
animation properties other than animation-name first, paused is
being set as the value of the animation-play-state property:

#anotherFailedAnimation {
    animation: running 2s ease-in-out forwards;
}

The preceding code snippet is the equivalent to:

#anotherFailedAnimation {
    animation-name: none;
    animation-duration: 2s;
    animation-delay: 0s;
    animation-timing-function: ease-in-out;
    animation-iteration-count: 1;
    animation-fill-mode: forwards;
    animation-direction: normal;
    animation-play-state: running;
}

The developer probably has a keyframe animation called running. The
browser, however, sees the term and assigns it to the
animation-play-state property rather than the animation-name
property. With no animation-name declared, there is no animation
attached to the element. The way to get around this would be:

#aSuccessfulIfInadvisableAnimation {
    animation: running 2s ease-in-out forwards running;
}

This will apply the first running to animation-play-state, and the second running to animation-name. Again: this is not advised. The potential for confusion and error is too great.

In light of all this, animation: 2s 3s 4s; may seem valid, as if the
following were being set:

#invalidName {
    animation-name: 4s;
    animation-duration: 2s;
    animation-delay: 3s;
}

But as was mentioned in “Setting Up Keyframe Animations”, 4s is not a
valid identifier. Identifiers cannot start with a digit unless escaped.
For this animation to be valid, it would have to be written as
animation: 2s 3s \4s;

To attach multiple animations to a single element or pseudo-element,
comma-separate the animations:

.snowflake {
  animation: 3s ease-in 200ms 32 forwards falling,
             1.5s linear 200ms 64 spinning;
}

The snowflake will fall while spinning for 96 seconds, spinning twice
during each 3-second fall. At the end of the last animation cycle, the
snowflake will stay fixed on the 100% keyframe of the falling animation. We declared six of the eight animation properties for
the falling animation and five for the spinning animation, separating the
two animations with a comma.

While you’ll most often see the animation name as the first value—it’s easier to read that way, because of the issue with animation
property keywords being valid keyframe identifiers—it is not a best
practice. That is why we put the animation name at the end.

To sum up: it is a fine idea to use the animation shorthand. Just
remember that the placements of the duration, delay, and name within that
shorthand are important, and omitted values will be set to their default
values. Also, it is a good idea to not use any animation keywords as
your identifier.

Animation, Specificity, and Precedence Order

In terms of specificity, the cascade, and which property values get
applied to an element, animations (as of late 2017) incorrectly supersede all other values in the cascade.

Specificity and !important

In general, the weight of a property attached with an ID selector 1-0-0
should take precedence over a property applied by an element selector
0-0-1. However, if that property value was changed via a keyframe
animation, it will be applied as if that property/value pair were added
as an inline style.

The current behavior in all browsers that support
animation is as if the property values set by keyframes
were declared inline with an
added !important—as if they were something like <div style="keyframe-property: value !important">. This is wrong, according to the specifications. The
animation specification states “animations override all normal rules,
but are overridden by !important rules.” This is a bug in the late 2017
implementations and should be resolved eventually. Or, perhaps, the specification will change.

That being said, don’t include !important within your animation
declaration block; this use is invalid, and
the property/value combination upon which it is declared will be ignored.

Animation Order

If there are multiple animations specifying values for the same
property, the property value from the last animation applied will
override the previous animations:

#colorchange {
  animation-name: red, green, blue;
  animation-duration: 11s, 9s, 6s;
}

In this code example, if red, green, and blue are all keyframe
animations that change the color property to their respective names,
once the animation-name and animation-duration properties are
applied to #colorchange, for the first six seconds, the property values in
blue will take precedence, then green for three seconds, then red for two seconds,
before returning to default property values. In this scenario, if the blue @keyframe animation does not include the color property in the 0% keyframe, the color will be taken from the animation named green, the animation named red, or the element’s currentColor value, in that order. The same is true for an omitted 100% keyframe.

The default properties of an element are not impacted before the
animation starts, and the properties return to their original values
after the animation ends unless an animation-fill-mode value other
than the default none has been set. If animation-fill-mode: both
were added to the mix, the color would always be blue, as the last
animation, or blue, overrides the previous green animation, which
overrides the red first animation.

Animation Iteration and display: none;

If the display property is set to none on an element, any animation
iterating on that element or its descendants will cease, as if the
animation were detached from the element. Updating the display
property back to a visible value will reattach all the animation
properties, restarting the animation from scratch:

.snowflake {
  animation: spin 2s linear 5s 20;
}

In this case, the snowflake will spin 20 times; each spin takes 2 seconds, with the
first spin starting after 5 seconds. If the snowflake element’s
display property gets set to none after 15 seconds, it would have
completed 5 spins before disappearing (after getting through the 5-second delay, then executing 5 spins at 2
seconds each). If the snowflake display property changes back to
anything other than none, the animation starts from scratch: a 5-second delay will elapse again before it starts spinning 20 times. It
makes no difference how many animation cycles iterated before it
disappeared from view the first time.

Animation and the UI Thread

CSS animations have the lowest priority on the user interface (UI) thread. If you attach
multiple animations on page load with positive values for
animation-delay, the delays expire as specified, but the animations
may not begin until the UI thread is available to animate.

Assume the following:

  • The animations all require the UI thread (that is, they aren’t on the GPU as
    described in “Animation chaining”).

  • You have 20
    animations with the animation-delay property
    set to 1s, 2s, 3s, 4s, and so on in order to start each subsequent animation one second after the previous animation.

  • The document or application takes
    a long time to load, with 11 seconds between the time the animated elements
    were drawn to the page and the time the JavaScript finished being
    downloaded, parsed, and executed.

Given all that, the delays of the first 11 animations
will have expired once the UI thread is available,
and those first 11 animations will all commence simultaneously.
The remaining animations will each then begin animating at
one-second intervals.

Seizure and Vestibular Disorders

Warning

While you can use animations to create ever-changing content, dynamically changing content can lead to seizures in some users. Always keep
this in mind, and ensure the accessibility of your website to
people with epilepsy and other seizure disorders.

We don’t usually start a section with a warning, but in this case, it’s warranted. Visual change, especially rapid visual change, can trigger medical emergencies in users who are prone to seizures. They can also cause severe unease in users who are prone to vestibular disorder (motion sickness).

As this book was going to press in late 2017, a new media query was being deployed in browsers: prefers-reduced-motion. This allows authors to apply styles when the user has a “Reduce motion” or similar preference set for their browser or device. Strongly consider an approach such as this:

@media (prefers-reduced-motion) {
  * {animation: none !important; transition: none !important;}
}

This disables all animations and transitions, assuming no other !important animations are specified (and they shouldn’t be). This is not a nuanced or perfect solution, but it’s a first step. You can invert this approach by segregating all of your animations and transitions in a media block for those who do not have motion reduction enabled, like this:

@media not (prefers-reduced-motion) {
  /* all animations and transitions */
}

Not all animations are dangerous or disorienting, and it may be necessary to have at least some animations for all users. In such cases, use prefers-reduced-motion to tone down animations that are essential to understanding of the UI, and to switch off those that are essentially decorative.

Animation Events and Prefixing

Let’s recap animation-related events we can access with DOM event
listeners, and what prefixing may be required when using them.

animationstart

The animationstart event occurs at the start of the animation. If
there is an animation-delay, this event will fire once the delay
period has expired. If there is no delay, the animationstart event occurs
when the animation is applied to the element. Even if there are no
iterations, the animationstart event still occurs. If there are
multiple animations attached to an element, an animationstart event
will occur for each of the applied valid keyframe animations: generally,
one animationstart for each valid animation-name identifier present:

#colorchange {
  animation: red, green, blue;
}

In this example, as long as the red, green, and blue keyframe
animations are valid, while the animations will not be perceptible (as
the default duration of 0s is set on each), there will be three
animationstart events thrown: one for each animation name.

If the browser requires the -webkit- prefix for the animation
properties—basically, Safari 8 and earlier and Android 4.4.4 and older—the event is written as webkitAnimationStart instead of
animationstart. Note the -webkit- prefix and the camelCasing. It is
best to default to the unprefixed syntax and fall back to the prefixed
version only when the unprefixed is unavailable.

animationend

The animationend event occurs at the conclusion of the last animation.
It only occurs once per applied animation: if an element has three
animations applied to it, like in the earlier #colorchange example, the
animationend event will occur three times, at the end of the animation. In the example, there was no duration for any of the
animations; however, the animationend event timing is usually
equivalent to the result of the following equation:

(animation-duration * animation-iteration-count) + animation-delay = time

Even if there are no iterations, the animationend event still occurs once
for each animation applied. If the animation-iteration-count is set to
infinite, the animationend event never occurs.

If the browser requires the -webkit- prefix for the animation
properties, the event is written as webkitAnimationEnd instead of
animationend.

animationiteration

The animationiteration event occurs at the end of each iteration of an
animation, before the start of the next iteration. If there are no
iterations, or the iteration count is less than or equal to one, the
animationiteration event never occurs. If the iteration count is
infinite, the animationiteration event occurs ad infinitum, unless
there is no duration set or the duration is 0s.

Unlike the animationstart and animationend events, which each occur
once for each animation name, the animationiteration event can occur
multiple times or no times per animation name, depending on how many
iterations occur. Note that the event happens between animation cycles and
will not occur at the same time as an animationend event. In other
words, if the animation-iteration-count is an integer, the number of
animationiteration events that occur is generally one less that the value of the
animation-iteration-count property, as long as the absolute value of any negative delay is less than the duration.

Printing Animations

While not actually “animating” on a printed piece of paper, when an
animated element is printed, the relevant property values will be
printed. You can’t see the element animating on a piece of
paper, but if the animation caused an element to have a border-radius of
50%, the printed element will have a border-radius of 50%.

1 For a detailed (and lovely) illustration of how cubic Bézier curves are actually constructed, we highly recommend the video “Cubic Bezier Curves – Under the Hood”.

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