HTML & CSS
Article
By Guy Routledge

AtoZ CSS Screencast: Keyframe Animations

By Guy Routledge

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This screencast is a part of our AtoZ CSS Series. You can find other entries to the series here.

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Transcript

Animation used to be the realm of JavaScript.

Now, in modern browsers, we can animate elements using CSS.

The @keyframes block and animation properties allow us to specify what gets animated and when.

In this episode we’ll look at the kind of animations that are suited to CSS, the concept of specifying a series of @keyframes and finally the way these keyframes are combined with animation settings to bring the page to life.

Animations in CSS

As CSS animations don’t have the deepest of browser support, they are best suited to visual flair rather than being a key part of the page content or design.

Animations can run 1 or more times or loop infinitely. It’s also possible to add multiple animations to the same element.

Animations can be triggered in CSS as soon as the page loads, after a delay or via some kind of state based change like :hover, :focus, or :active which we looked at in Episode 8: Hover.

CSS animations can also be started and stopped in Javascript by toggling the animation-play-state property. We’ll look at the other animation properties and syntax a bit later on.

Keyframes

In order to animate an element (or selection of elements), we need to specify a series of keyframes.

The most basic form of keyframe animation goes from one set of styles at the beginning to another set of styles at the end, over a certain amount of time.

During the animation, the styles between keyframes are automatically calculated by the browser – a process known as tweening.

Each keyframe is defined as a style block of CSS properties that will be applied to any element that uses that set of keyframes.

@keyframes moveLeft {
  from {left: 0;}
  to   {left: 500px;}
}

Given this set of keyframes, the starting keyframe of the animation will look like this; and the ending one will look like this.

@keyframes grow {
  0%   {font-size: 20px;}
  75%  {font-size: 100px;}
  100% {font-size: 10px;}
}

Additional keyframes can be specified using a percentages syntax. If the animation duration was 10 seconds, over the first 7.5 seconds, the font-size of the element would grow to 100px and then over the next 2.5 seconds, it would shrink back down to 10px. You can specify as many properties as you like for each keyframe.

Animation

When the @keyframes have been defined, they are ready to be used in conjunction with the animation-name property.

There are a series of animation properties to configure your animation as needed:

  • animation-name specifies the block of @keyframes to use
  • animation-duration specified the time the animation lasts
  • animation-delay specifies any delay before the animation starts
  • animation-iteration-count specifies the number of times to repeat
  • animation-direction specifies the direction; animations can play forwards (normal), in reverse or alternate back and forth
  • animation-play-state allows the animation to be paused and resumed
  • animation-timing-function determines an acceleration curve of how the animation plays between keyframes
  • animation-fill-mode determines how styles are applied before and after the animation

These 8 properties can be combined into a shorthand animation property as follows:

.box {
  animation: name duration delay count direction play-state timing fill-mode;
}

The only required properties for an animation to be visible at least once are: animation-name and animation-duration.

Bouncing ball animation

Let’s look at a practical example.

We can create a ball using an equal width and height box with border-radius set to 100%. We can make the ball bounce up and down by asbolutely positioning it and animating the top or bottom values over time.

We can give the ball a bit more realism by squashing it at the bottom of the animation before having it travel back up, at a slightly slower speed.

.ball {
  position: absolute;
  width: 100px;
  height: 100px;
  border-radius: 100%;
  animation: bounce 3s linear infinite;
}
@keyframes bounce {
  0% {bottom: 100%;}
  25% {
    bottom: 0;
    width: 100px;
    height: 100px;
  }
  30% {
    bottom: 0;
    height: 50px;
    width: 110px;
  }
  35% {
    bottom: 0;
    width: 100px;
    height: 100px;
  }
  70% {
    bottom: 100%;
  }
  100% {
    bottom: 100%;
  }
}

We can make the ball move across the screen by adding a second animation that animates the left property of the ball. These can be comma separated, so they are both applied to the same element.

To make it appear as though the ball bounces slowly across the screen, we can increase the duration of this second animation.

@keyframes moveLeft {
  from {left: 0;}
  to   {left: 100%;}
}
.ball {
  animation: bounce 3s linear, moveLeft 12s linear infinite;
}

Browser support

CSS animations are not supported in IE9 or below or Opera Mini. In IE10, IE11 and Firefox, the @keyframes and animation properties are unprefixed but -webkit prefixes are needed everywhere else.

Watch out for our Quick Tip article coming soon!

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