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.

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.

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.

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

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;
}

<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>

<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>

<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>

.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);
    }
}

.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);
  }
}

.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);
    }
}

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);

});

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

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.

@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);
  }
}

.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;
}

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.

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.

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.