animation

The animation property in CSS can be used to animate many other CSS properties such as color, background-color, height, or width. Each animation needs to be defined with the @keyframes at-rule which is then called with the animation property, like so:

.element {
  animation: pulse 5s infinite;
}

@keyframes pulse {
  0% {
    background-color: #001F3F;
  }
  100% {
    background-color: #FF4136;
  }
}

Each @keyframes at-rule defines what should happen at specific moments during the animation. For example, 0% is the beginning of the animation and 100% is the end. These keyframes can then be controlled either by the shorthand animation property, or its eight sub-properties, to give more control over how those keyframes should be manipulated.

Sub-properties

• animation-name: declares the name of the @keyframes at-rule to manipulate.
• animation-duration: the length of time it takes for an animation to complete one cycle.
• animation-timing-function: establishes preset acceleration curves such as ease or linear.
• animation-delay: the time between the element being loaded and the start of the animation sequence (cool examples).
• animation-direction: sets the direction of the animation after the cycle. Its default resets on each cycle.
• animation-iteration-count: the number of times the animation should be performed.
• animation-fill-mode: sets which values are applied before/after the animation.
  For example, you can set the last state of the animation to remain on screen, or you can set it to switch back to before when the animation began.
• animation-play-state: pause/play the animation.

These sub-properties can then be used like so:

@keyframes stretch {
  /* declare animation actions here */
}

.element {
  animation-name: stretch;
  animation-duration: 1.5s; 
  animation-timing-function: ease-out; 
  animation-delay: 0s;
  animation-direction: alternate;
  animation-iteration-count: infinite;
  animation-fill-mode: none;
  animation-play-state: running; 
}

/*
  is the same as:
*/

.element {
  animation: 
    stretch
    1.5s
    ease-out
    0s
    alternate
    infinite
    none
    running;
}

Here’s a full list of which values each of these sub-properties can take:

animation-timing-function	ease, ease-out, ease-in, ease-in-out, linear, cubic-bezier(x1, y1, x2, y2) (e.g. cubic-bezier(0.5, 0.2, 0.3, 1.0))
animation-duration	Xs or Xms
animation-delay	Xs or Xms
animation-iteration-count	X
animation-fill-mode	forwards, backwards, both, none
animation-direction	normal, alternate
animation-play-state	paused, running, running

Multiple steps

If an animation has the same starting and ending properties, it’s useful to comma-separate the 0% and 100% values inside @keyframes:

@keyframes pulse {
  0%, 100% {
    background-color: yellow;
  }
  50% {
    background-color: red;
  }
}

Multiple animations

You can comma-separate the values to declare multiple animations on a selector as well. In the example below, we want to change the color of the circle in a @keyframe whilst also nudging it from side to side with another.

.element {
  animation: 
    pulse 3s ease infinite alternate, 
    nudge 5s linear infinite alternate;
}

Performance

Animating most properties is a performance concern, so we should proceed with caution before animating any property. However, there are certain combinations that can be animated safely:

• transform: translate()
• transform: scale()
• transform: rotate()
• opacity

Which properties can be animated?

MDN has a list of CSS properties which can be animated. Animatable properties tend to colors and numbers. An example a non-animatable property is background-image.

Browser support

More information

• Using CSS animations (MDN)
• CSS Animations Level 1 (W3C specification)
• CSS Animation Shorthand Property (DigitalOcean)
• Web animation at work (A List Apart)
• Five ways to animate responsibly (24 Ways)

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