January 20, 2024

CSS-animations

CSS animations make it possible to do simple animations without JavaScript at all.

JavaScript can be used to control CSS animations and make them even better, with little code.

CSS transitions

The idea of CSS transitions is simple. We describe a property and how its changes should be animated. When the property changes, the browser paints the animation.

That is, all we need is to change the property, and the fluid transition will be done by the browser.

For instance, the CSS below animates changes of background-color for 3 seconds:

          .animated {
  transition-property: background-color;
  transition-duration: 3s;
}
        

Now if an element has .animated class, any change of background-color is animated during 3 seconds.

Click the button below to animate the background:

<button id="color">Click me</button>

<style>
  #color {
    transition-property: background-color;
    transition-duration: 3s;
  }
</style>

<script>
  color.onclick = function() {
    this.style.backgroundColor = 'red';
  };
</script>

There are 4 properties to describe CSS transitions:

transition-property
transition-duration
transition-timing-function
transition-delay

We’ll cover them in a moment, for now let’s note that the common transition property allows declaring them together in the order: property duration timing-function delay, as well as animating multiple properties at once.

For instance, this button animates both color and font-size:

<button id="growing">Click me</button>

<style>
#growing {
  transition: font-size 3s, color 2s;
}
</style>

<script>
growing.onclick = function() {
  this.style.fontSize = '36px';
  this.style.color = 'red';
};
</script>

Now, let’s cover animation properties one by one.

transition-property

In transition-property, we write a list of properties to animate, for instance: left, margin-left, height, color. Or we could write all, which means “animate all properties”.

Do note that, there are properties which can not be animated. However, most of the generally used properties are animatable.

transition-duration

In transition-duration we can specify how long the animation should take. The time should be in CSS time format: in seconds s or milliseconds ms.

transition-delay

In transition-delay we can specify the delay before the animation. For instance, if transition-delay is 1s and transition-duration is 2s, then the animation starts 1 second after the property change and the total duration will be 2 seconds.

Negative values are also possible. Then the animation is shown immediately, but the starting point of the animation will be after given value (time). For example, if transition-delay is -1s and transition-duration is 2s, then animation starts from the halfway point and total duration will be 1 second.

Here the animation shifts numbers from 0 to 9 using CSS translate property:

Result

script.js

style.css

index.html

stripe.onclick = function() {
  stripe.classList.add('animate');
};

#digit {
  width: .5em;
  overflow: hidden;
  font: 32px monospace;
  cursor: pointer;
}

#stripe {
  display: inline-block
}

#stripe.animate {
  transform: translate(-90%);
  transition-property: transform;
  transition-duration: 9s;
  transition-timing-function: linear;
}

<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  Click below to animate:

  <div id="digit"><div id="stripe">0123456789</div></div>

  <script src="script.js"></script>
</body>

</html>

The transform property is animated like this:

          #stripe.animate {
  transform: translate(-90%);
  transition-property: transform;
  transition-duration: 9s;
}
        

In the example above JavaScript adds the class .animate to the element – and the animation starts:

stripe.classList.add('animate');

We could also start it from somewhere in the middle of the transition, from an exact number, e.g. corresponding to the current second, using a negative transition-delay.

Here if you click the digit – it starts the animation from the current second:

Result

script.js

style.css

index.html

stripe.onclick = function() {
  let sec = new Date().getSeconds() % 10;
  stripe.style.transitionDelay = '-' + sec + 's';
  stripe.classList.add('animate');
};

#digit {
  width: .5em;
  overflow: hidden;
  font: 32px monospace;
  cursor: pointer;
}

#stripe {
  display: inline-block
}

#stripe.animate {
  transform: translate(-90%);
  transition-property: transform;
  transition-duration: 9s;
  transition-timing-function: linear;
}

<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  Click below to animate:
  <div id="digit"><div id="stripe">0123456789</div></div>

  <script src="script.js"></script>
</body>
</html>

JavaScript does it with an extra line:

          stripe.onclick = function() {
  let sec = new Date().getSeconds() % 10;
  // for instance, -3s here starts the animation from the 3rd second
  stripe.style.transitionDelay = '-' + sec + 's';
  stripe.classList.add('animate');
};
        

transition-timing-function

The timing function describes how the animation process is distributed along its timeline. Will it start slowly and then go fast, or vice versa.

It appears to be the most complicated property at first. But it becomes very simple if we devote a bit time to it.

That property accepts two kinds of values: a Bezier curve or steps. Let’s start with the curve, as it’s used more often.

Bezier curve

The timing function can be set as a Bezier curve with 4 control points that satisfy the conditions:

First control point: (0,0).
Last control point: (1,1).
For intermediate points, the values of x must be in the interval 0..1, y can be anything.

The syntax for a Bezier curve in CSS: cubic-bezier(x2, y2, x3, y3). Here we need to specify only 2nd and 3rd control points, because the 1st one is fixed to (0,0) and the 4th one is (1,1).

The timing function describes how fast the animation process goes.

The x axis is the time: 0 – the start, 1 – the end of transition-duration.
The y axis specifies the completion of the process: 0 – the starting value of the property, 1 – the final value.

The simplest variant is when the animation goes uniformly, with the same linear speed. That can be specified by the curve cubic-bezier(0, 0, 1, 1).

Here’s how that curve looks:

…As we can see, it’s just a straight line. As the time (x) passes, the completion (y) of the animation steadily goes from 0 to 1.

The train in the example below goes from left to right with the permanent speed (click it):

Result

style.css

index.html

.train {
  position: relative;
  cursor: pointer;
  width: 177px;
  height: 160px;
  left: 0;
  transition: left 5s cubic-bezier(0, 0, 1, 1);
}

<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  <img class="train" src="https://js.cx/clipart/train.gif" onclick="this.style.left='450px'">

</body>

</html>

The CSS transition is based on that curve:

          .train {
  left: 0;
  transition: left 5s cubic-bezier(0, 0, 1, 1);
  /* click on a train sets left to 450px, thus triggering the animation */
}
        

…And how can we show a train slowing down?

We can use another Bezier curve: cubic-bezier(0.0, 0.5, 0.5 ,1.0).

The graph:

As we can see, the process starts fast: the curve soars up high, and then slower and slower.

Here’s the timing function in action (click the train):

Result

style.css

index.html

.train {
  position: relative;
  cursor: pointer;
  width: 177px;
  height: 160px;
  left: 0px;
  transition: left 5s cubic-bezier(0.0, 0.5, 0.5, 1.0);
}

<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  <img class="train" src="https://js.cx/clipart/train.gif" onclick="this.style.left='450px'">

</body>

</html>

CSS:

          .train {
  left: 0;
  transition: left 5s cubic-bezier(0, .5, .5, 1);
  /* click on a train sets left to 450px, thus triggering the animation */
}
        

There are several built-in curves: linear, ease, ease-in, ease-out and ease-in-out.

The linear is a shorthand for cubic-bezier(0, 0, 1, 1) – a straight line, which we described above.

Other names are shorthands for the following cubic-bezier:

`ease`^*	`ease-in`	`ease-out`	`ease-in-out`
`(0.25, 0.1, 0.25, 1.0)`	`(0.42, 0, 1.0, 1.0)`	`(0, 0, 0.58, 1.0)`	`(0.42, 0, 0.58, 1.0)`

* – by default, if there’s no timing function, ease is used.

So we could use ease-out for our slowing down train:

          .train {
  left: 0;
  transition: left 5s ease-out;
  /* same as transition: left 5s cubic-bezier(0, .5, .5, 1); */
}
        

But it looks a bit differently.

A Bezier curve can make the animation exceed its range.

The control points on the curve can have any y coordinates: even negative or huge ones. Then the Bezier curve would also extend very low or high, making the animation go beyond its normal range.

In the example below the animation code is:

          .train {
  left: 100px;
  transition: left 5s cubic-bezier(.5, -1, .5, 2);
  /* click on a train sets left to 450px */
}
        

The property left should animate from 100px to 400px.

But if you click the train, you’ll see that:

First, the train goes back: left becomes less than 100px.
Then it goes forward, a little bit farther than 400px.
And then back again – to 400px.

Result

style.css

index.html

.train {
  position: relative;
  cursor: pointer;
  width: 177px;
  height: 160px;
  left: 100px;
  transition: left 5s cubic-bezier(.5, -1, .5, 2);
}

<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  <img class="train" src="https://js.cx/clipart/train.gif" onclick="this.style.left='400px'">

</body>

</html>

Why it happens is pretty obvious if we look at the graph of the given Bezier curve:

We moved the y coordinate of the 2nd point below zero, and for the 3rd point we made it over 1, so the curve goes out of the “regular” quadrant. The y is out of the “standard” range 0..1.

As we know, y measures “the completion of the animation process”. The value y = 0 corresponds to the starting property value and y = 1 – the ending value. So values y<0 move the property beyond the starting left and y>1 – past the final left.

That’s a “soft” variant for sure. If we put y values like -99 and 99 then the train would jump out of the range much more.

But how do we make a Bezier curve for a specific task? There are many tools.

For instance, we can do it on the site https://cubic-bezier.com.
Browser developer tools also have special support for Bezier curves in CSS:
1. Open the developer tools with F12 (Mac: Cmd+Opt+I).
2. Select the Elements tab, then pay attention to the Styles sub-panel at the right side.
3. CSS properties with a word cubic-bezier will have an icon before this word.
4. Click this icon to edit the curve.

Steps

The timing function steps(number of steps[, start/end]) allows splitting an transition into multiple steps.

Let’s see that in an example with digits.

Here’s a list of digits, without any animations, just as a source:

Result

style.css

index.html

#digit {
  border: 1px solid red;
  width: 1.2em;
}

#stripe {
  display: inline-block;
  font: 32px monospace;
}

<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  <div id="digit"><div id="stripe">0123456789</div></div>

</body>
</html>

In the HTML, a stripe of digits is enclosed into a fixed-length <div id="digits">:

          <div id="digit">
  <div id="stripe">0123456789</div>
</div>
        

The #digit div has a fixed width and a border, so it looks like a red window.

We’ll make a timer: the digits will appear one by one, in a discrete way.

To achieve that, we’ll hide the #stripe outside of #digit using overflow: hidden, and then shift the #stripe to the left step-by-step.

There will be 9 steps, a step-move for each digit:

          #stripe.animate  {
  transform: translate(-90%);
  transition: transform 9s steps(9, start);
}
        

The first argument of steps(9, start) is the number of steps. The transform will be split into 9 parts (10% each). The time interval is automatically divided into 9 parts as well, so transition: 9s gives us 9 seconds for the whole animation – 1 second per digit.

The second argument is one of two words: start or end.

The start means that in the beginning of animation we need to make the first step immediately.

In action:

Result

style.css

index.html

#digit {
  width: .5em;
  overflow: hidden;
  font: 32px monospace;
  cursor: pointer;
}

#stripe {
  display: inline-block
}

#stripe.animate {
  transform: translate(-90%);
  transition-property: transform;
  transition-duration: 9s;
  transition-timing-function: steps(9, start);
}

<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  Click below to animate:

  <div id="digit"><div id="stripe">0123456789</div></div>

  <script>
    digit.onclick = function() {
      stripe.classList.add('animate');
    }
  </script>


</body>

</html>

A click on the digit changes it to 1 (the first step) immediately, and then changes in the beginning of the next second.

The process is progressing like this:

0s – -10% (first change in the beginning of the 1st second, immediately)
1s – -20%
…
8s – -90%
(the last second shows the final value).

Here, the first change was immediate because of start in the steps.

The alternative value end would mean that the change should be applied not in the beginning, but at the end of each second.

So the process for steps(9, end) would go like this:

0s – 0 (during the first second nothing changes)
1s – -10% (first change at the end of the 1st second)
2s – -20%
…
9s – -90%

Here’s steps(9, end) in action (note the pause before the first digit change):

Result

style.css

index.html

#digit {
  width: .5em;
  overflow: hidden;
  font: 32px monospace;
  cursor: pointer;
}

#stripe {
  display: inline-block
}

#stripe.animate {
  transform: translate(-90%);
  transition-property: transform;
  transition-duration: 9s;
  transition-timing-function: steps(9, end);
}

<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  Click below to animate:

  <div id="digit"><div id="stripe">0123456789</div></div>

  <script>
    digit.onclick = function() {
      stripe.classList.add('animate');
    }
  </script>


</body>

</html>

There are also some pre-defined shorthands for steps(...):

step-start – is the same as steps(1, start). That is, the animation starts immediately and takes 1 step. So it starts and finishes immediately, as if there were no animation.
step-end – the same as steps(1, end): make the animation in a single step at the end of transition-duration.

These values are rarely used, as they represent not a real animation, but rather a single-step change. We mention them here for completeness.

Event: “transitionend”

When the CSS animation finishes, the transitionend event triggers.

It is widely used to do an action after the animation is done. Also we can join animations.

For instance, the ship in the example below starts to sail there and back when clicked, each time farther and farther to the right:

The animation is initiated by the function go that re-runs each time the transition finishes, and flips the direction:

          boat.onclick = function() {
  //...
  let times = 1;

  function go() {
    if (times % 2) {
      // sail to the right
      boat.classList.remove('back');
      boat.style.marginLeft = 100 * times + 200 + 'px';
    } else {
      // sail to the left
      boat.classList.add('back');
      boat.style.marginLeft = 100 * times - 200 + 'px';
    }

  }

  go();

  boat.addEventListener('transitionend', function() {
    times++;
    go();
  });
};
        

The event object for transitionend has a few specific properties:

event.propertyName: The property that has finished animating. Can be good if we animate multiple properties simultaneously.
event.elapsedTime: The time (in seconds) that the animation took, without transition-delay.

Keyframes

We can join multiple simple animations together using the @keyframes CSS rule.

It specifies the “name” of the animation and rules – what, when and where to animate. Then using the animation property, we can attach the animation to the element and specify additional parameters for it.

Here’s an example with explanations:

<div class="progress"></div>

<style>
  @keyframes go-left-right {        /* give it a name: "go-left-right" */
    from { left: 0px; }             /* animate from left: 0px */
    to { left: calc(100% - 50px); } /* animate to left: 100%-50px */
  }

  .progress {
    animation: go-left-right 3s infinite alternate;
    /* apply the animation "go-left-right" to the element
       duration 3 seconds
       number of times: infinite
       alternate direction every time
    */

    position: relative;
    border: 2px solid green;
    width: 50px;
    height: 20px;
    background: lime;
  }
</style>

There are many articles about @keyframes and a detailed specification.

You probably won’t need @keyframes often, unless everything is in constant motion on your sites.

Performance

Most CSS properties can be animated, because most of them are numeric values. For instance, width, color, font-size are all numbers. When you animate them, the browser gradually changes these numbers frame by frame, creating a smooth effect.

However, not all animations will look as smooth as you’d like, because different CSS properties cost differently to change.

In more technical details, when there’s a style change, the browser goes through 3 steps to render the new look:

Layout: re-compute the geometry and position of each element, then
Paint: re-compute how everything should look like at their places, including background, colors,
Composite: render the final results into pixels on screen, apply CSS transforms if they exist.

During a CSS animation, this process repeats every frame. However, CSS properties that never affect geometry or position, such as color, may skip the Layout step. If a color changes, the browser doesn’t calculate any new geometry, it goes to Paint → Composite. And there are few properties that directly go to Composite. You can find a longer list of CSS properties and which stages they trigger at https://csstriggers.com.

The calculations may take time, especially on pages with many elements and a complex layout. And the delays are actually visible on most devices, leading to “jittery”, less fluid animations.

Animations of properties that skip the Layout step are faster. It’s even better if Paint is skipped too.

The transform property is a great choice, because:

CSS transforms affect the target element box as a whole (rotate, flip, stretch, shift it).
CSS transforms never affect neighbour elements.

…So browsers apply transform “on top” of existing Layout and Paint calculations, in the Composite stage.

In other words, the browser calculates the Layout (sizes, positions), paints it with colors, backgrounds, etc at the Paint stage, and then applies transform to element boxes that need it.

Changes (animations) of the transform property never trigger Layout and Paint steps. More than that, the browser leverages the graphics accelerator (a special chip on the CPU or graphics card) for CSS transforms, thus making them very efficient.

Luckily, the transform property is very powerful. By using transform on an element, you could rotate and flip it, stretch and shrink it, move it around, and much more. So instead of left/margin-left properties we can use transform: translateX(…), use transform: scale for increasing element size, etc.

The opacity property also never triggers Layout (also skips Paint in Mozilla Gecko). We can use it for show/hide or fade-in/fade-out effects.

Paring transform with opacity can usually solve most of our needs, providing fluid, good-looking animations.

For example, here clicking on the #boat element adds the class with transform: translateX(300px) and opacity: 0, thus making it move 300px to the right and disappear:

<img src="https://js.cx/clipart/boat.png" id="boat">

<style>
#boat {
  cursor: pointer;
  transition: transform 2s ease-in-out, opacity 2s ease-in-out;
}

.move {
  transform: translateX(300px);
  opacity: 0;
}
</style>
<script>
  boat.onclick = () => boat.classList.add('move');
</script>

Here’s a more complex example, with @keyframes:

<h2 onclick="this.classList.toggle('animated')">click me to start / stop</h2>
<style>
  .animated {
    animation: hello-goodbye 1.8s infinite;
    width: fit-content;
  }
  @keyframes hello-goodbye {
    0% {
      transform: translateY(-60px) rotateX(0.7turn);
      opacity: 0;
    }
    50% {
      transform: none;
      opacity: 1;
    }
    100% {
      transform: translateX(230px) rotateZ(90deg) scale(0.5);
      opacity: 0;
    }
  }
</style>

Summary

CSS animations allow smoothly (or step-by-step) animated changes of one or multiple CSS properties.

They are good for most animation tasks. We’re also able to use JavaScript for animations, the next chapter is devoted to that.

Limitations of CSS animations compared to JavaScript animations:

Merits

Simple things done simply.
Fast and lightweight for CPU.

Demerits

JavaScript animations are flexible. They can implement any animation logic, like an “explosion” of an element.
Not just property changes. We can create new elements in JavaScript as part of the animation.

In early examples in this chapter, we animate font-size, left, width, height, etc. In real life projects, we should use transform: scale() and transform: translate() for better performance.

The majority of animations can be implemented using CSS as described in this chapter. And the transitionend event allows JavaScript to be run after the animation, so it integrates fine with the code.

But in the next chapter we’ll do some JavaScript animations to cover more complex cases.

Tasks

Animate a plane (CSS)

Show the animation like on the picture below (click the plane):

The picture grows on click from 40x24px to 400x240px (10 times larger).
The animation takes 3 seconds.
At the end output: “Done!”.
During the animation process, there may be more clicks on the plane. They shouldn’t “break” anything.

Open a sandbox for the task.

CSS to animate both width and height:

          /* original class */

#flyjet {
  transition: all 3s;
}

/* JS adds .growing */
#flyjet.growing {
  width: 400px;
  height: 240px;
}
        

Please note that transitionend triggers two times – once for every property. So if we don’t perform an additional check then the message would show up 2 times.

Open the solution in a sandbox.

Animate the flying plane (CSS)

Modify the solution of the previous task Animate a plane (CSS) to make the plane grow more than its original size 400x240px (jump out), and then return to that size.

Here’s how it should look (click on the plane):

Take the solution of the previous task as the source.

We need to choose the right Bezier curve for that animation. It should have y>1 somewhere for the plane to “jump out”.

For instance, we can take both control points with y>1, like: cubic-bezier(0.25, 1.5, 0.75, 1.5).

The graph:

Open the solution in a sandbox.

Animated circle with callback

In the task Animated circle an animated growing circle is shown.

Now let’s say we need not just a circle, but to show a message inside it. The message should appear after the animation is complete (the circle is fully grown), otherwise it would look ugly.

In the solution of the task, the function showCircle(cx, cy, radius) draws the circle, but gives no way to track when it’s ready.

Add a callback argument: showCircle(cx, cy, radius, callback) to be called when the animation is complete. The callback should receive the circle <div> as an argument.

Here’s the example:

          showCircle(150, 150, 100, div => {
  div.classList.add('message-ball');
  div.append("Hello, world!");
});
        

Demo:

Take the solution of the task Animated circle as the base.

Open the solution in a sandbox.

Tutorial map

Comments

read this before commenting…

CSS-animations

CSS transitions

transition-property

transition-duration

transition-delay

transition-timing-function

Bezier curve

Steps

Event: “transitionend”

Keyframes

Performance

Summary

Tasks

Animate a plane (CSS)

Animate the flying plane (CSS)

Animated circle

Animated circle with callback

Comments

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