CSS Animations explained

The second way to animate your components is through CSS Animations. CSS Animations are like CSS Transitions, except they’re way more powerful.

You create a CSS Animation by defining an animation in @keyframes.

@keyframes animation-name {
  0% {
    margin-left: 0px;
  }

  50% {
    margin-right: 400px;
  }

  100% {
    margin-left: 0px;
  }
}

In the keyframes above, the 0%, 50% and 100% values are points on an animation timeline. It means the following:

  1. Begin the animation (0%) with margin-left set to 0px.
  2. Animate margin-left to 400px as the animation continues to the the middle of the animation duration (50%)
  3. Animate margin-left to 0px as the animation continues to the end (100%)

You can add any number of points on the @keyframes declaration. Each point you add should be a percentage value. (You can also substitute from for 0% and to for 100%).

@keyframes animation-name {
  0% {
    margin-left: 0px;
  }

  25% {
    margin-right: 200px;
  }

  50%, {
    margin-right: 400px;
  }

  75% {
    margin-right: 200px;
  }

  100% {
    margin-left: 0px;
  }
}

If your @keyframes points contain similar properties (like in the above case), you can stack the points together to compact the @keyframes declaration.

@keyframes animation-name {
  from, to {
    margin-left: 0px;
  }

  25%, 75% {
    margin-right: 200px;
  }

  50% {
    margin-right: 400px;
  }
}

Using the animation

You can use the animation you’ve created with the animation property. Like transition, animation is a short hand for a handful of animation- properties (8 in total).

.component {
  animation-name: name;
  animation-duration: duration;
  animation-timing-function: timing-function;
  animation-delay: delay;
  animation-iteration-count: count;
  animation-direction: direction;
  animation-fill-mode: fill-mode;
  animation-play-state: play-state;

  /* OR */
  animation: name
             duration
             timing-function
             delay
             iteration-count
             direction
             fill-mode
             play-state;
}

animation-name is the name of the animation you created with the @keyframes syntax. You can include more than one animation by separating them with commas.

animation-duration is the duration of the animation. It is written in seconds with the s suffix, like 3s. It is also a required value.

animation-timing-function is the timing-function used for the animation. It has the same syntax as a transition-timing-function. If left out, it defaults to ease.

animation-delay is the delay before starting the animation. It creates a delay for every iteration of the animation and is written in seconds with the s suffix, like 3s. If left out, it defaults to 0s.

animation-iteration-count tells CSS how many times you want the animation to occur. It takes in a number value. If you want the animation to loop infinitely (until you say stop), use infinite. If left out, it defaults to 1.

animation-direction is the direction of the animation. More on animation-direction below.

animation-fill-mode tells CSS what to do when the animation ends. More on animation-fill-mode below.

animation-play-state determines the state of the animation. It can either be running (which means the animation is playing) or paused. If left out, it defaults to running.

Animation-direction

animation-direction tells CSS to play the animation from 0% to 100% or vice versa.

If the animation-direction is set to normal, the animation plays from 0% to 100%.

If the animation-direction is set to reverse, the animation plays from 100% to 0%.

If the animation-direction is set to alternate, the animation plays from 0% to 100% first, then plays from 100% to 0%, and from 0% to 100% again, until the animating-iteration-count runs out.

If the animation-direction is set to alternate-reverse, the animation plays from 100% to 0% first, then plays from 0% to 100%, and from 100% to 0% again, until the animating-iteration-count runs out.

See the Pen Animation direction demo by Zell Liew (@zellwk) on CodePen.

Animation-fill-mode

animation-fill-mode tells CSS how to style the animated element when the animation ends. It can take four possible values: none, forwards, backwards and both.

(If the animation-iteration-count is infinite, this property does nothing).

animation-fill-mode: none; tells CSS to style the animated element CSS rules that apply to it. Nothing from the @keyframes declaration should affect the animation when it ends.

animation-fill-mode: forwards; tells CSS to style the animated element such that it contains styles from the last @keyframes it encountered. If the animation goes in the normal direction, CSS will style the element with properties from the 100% step. If the animation goes in the reverse direction, CSS styles the element with properties from the 0% step.

animation-fill-mode: backwards; tells CSS to style the animated element according to where it originated. In practice, this looks exactly the same as none. I don’t see a good use case for it.

animation-fill-mode: both; tells CSS to apply properties in both the forwards and backwards direction. In practice, this works exactly like forwards most of the time. I don’t see a good use case for it too.

Of the four different fill-modes, the one you’ll most likely use is forwards.

See the Pen Animation fill-mode demo by Zell Liew (@zellwk) on CodePen.

Animating two or more properties.

You can animate two or more properties from the same element by separating the animation-name property with commas, just like how you’d do it with transition-property.

.selector {
  animation-name: slideIn, fadeIn;
  animation-duration: 2s;
}

The syntax is exactly the same as transitioning multiple properties, so I’m confident you know how to take it from here :)

Vary your @keyframes points

Setting @keyframes point values to standard, divisible values like 0%, 25%, 50%, 75% and 100% can make your animation dull and boring – because its predictable.

You can vary your @keyframe point values so your animation becomes a bit more life-like, like the heartbeat example here:

@keyframes heartbeat {
  0% { transform: scale(1); }
  35% { transform: scale(1.3); }
  50% { transform: scale(1); }
  65% { transform: scale(1.2); }
  80% { transform: scale(1); }
}

See the Pen Heartbeat simple by Zell Liew (@zellwk) on CodePen.

Points and timing

It’s hard to time your CSS animation with other elements on the page at the same time.

For example, let’s say you decide the animation should last for three seconds. You go ahead and calculate your points. In three seconds, if you want a specific part of the animation to take one second, you need to divide 100% by three. That means 33.333%.

Later, you decide to change the entire animation to two seconds, but you still want the same part to be one second. Now you’re screwed. You need to redo your entire @keyframes sequence because 33.333% now means 0.67 seconds instead.

😡.

I have to confess here, this is why I don’t use CSS animations much. Whenever I have a complicated animation I want to create, I’ll use JavaScript instead. Animating with JavaScript is much easier once you get used to the syntax.

Playing/pausing your animation with CSS or JavaScript

CSS animations play immediately when the page loads. If you want to play/pause your CSS animation on an event, you need to change the animation-play-state.

.heart {
  animation-play-state: running;
}

.heart:hover {
  animation-play-state: paused;
}

Mouse over the heart below and you’ll see a paused animation:

See the Pen Heartbeat (pause on hover) by Zell Liew (@zellwk) on CodePen.

IMPORTANT! Always pause your CSS animations when you don’t need them! This stops the animations from moving, which saves precious computing power that can be used for other things.

When to use CSS Animations

Now, this is the big question. You’ve learned so much about CSS animations. When should you use it?

Although CSS animations are useful, I recommend it only one or more if the following scenarios are met.

  1. You need a complex animation that CSS Transitions cannot provide you with (like the heartbeat animation for example)
  2. You need to transition more than 2 properties for a single element.
  3. The animation is relatively simple. It should not be more than 4 steps. Any more, you’re in for a headache.
  4. You want to trigger the animation when the screen loads (without listening for any JavaScript event).

If the animation becomes complicated enough to exceed 4 steps, I recommend you animate your component with JavaScript. It’s far easier to calculate and synchronize timings with JavaScript.

Wrapping up

CSS Animations are kind of a powered-up version of CSS Transitions. They allow you to create multi-step transitions through the @keyframes syntax.

To use an animation you created, you can specify the animation in animation or animation-property.

CSS Animations are great for animations that are relatively simple, because of the way @keyframes is structured. If you want to create a more complex animation, I highly recommend Javascript to create your animation instead.

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