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Web animation tips

Warning: this entry is a work-in-progress and incomplete. That said, it's still a useful reference to me which is why I've published it. I’ll flesh it out soon!

There are lots of different strands of web development. You try your best to be good at all of them, but there’s only so much time in the day! Animation is an area where I know a little but would love to know more, and from a practical perspective I’d certainly benefit from having some road-ready solutions to common challenges. As ever I want to favour web standards over libraries where possible, and take an approach that’s lean, accessible, progressively-enhanced and performance-optimised.

Here’s my attempt to break down web animation into bite-sized chunks for ocassional users like myself.

Defining animation

Animation lets us make something visually move between different states over a given period of time.

Benefits of animation

Animation is a good way of providing visual feedback, teaching users how to use a part of the interface, or adding life to a website and making it feel more “real”.

Simple animation with transition properties

CSS transition is great for simple animations triggered by an event.

We start by defining two different states for an element—for example opacity:1 and opacity:0—and then transition between those states.

The first state would be in the element’s starting styles (either defined explicitly or existing implicitly based on property defaults) and the other in either its :hover or :focus styles or in a class applied by JavaScript following an event.

Without the transition the state change would still happen but would be instantaneous.

You’re not limited to only one property being animated and might, for example, transition between different opacity and transform states simultaneously.

Here’s an example “rise on hover” effect, adapted from Stephanie Eckles’s Smol CSS.

<div class="u-animate u-animate--rise">
  <span>rise</span>
</div>
.u-animate > * {
  --transition-property: transform;
  --transition-duration: 180ms;
  transition: var(--transition-property) var(--transition-duration) ease-in-out;
}

.u-animate--rise:hover > * {
  transform: translateY(-25%);
}

Note that:

  1. using custom properties makes it really easy to transition a different property than transform without writing repetitious CSS.
  2. we have a parent and child (<div> and <span> respectively in this example) allowing us to avoid the accidental flicker which can occur when the mouse is close to an animatable element’s border by having the child be the effect which animates when the trigger (the parent) is hovered.

Complex animations with animation properties

If an element needs to animate automatically (perhaps on page load or when added to the DOM), or is more complex than a simple A to B state change, then a CSS animation may be more appropriate than transition. Using this approach, animations can:

  • run automatically (you don’t need an event to trigger a state change)
  • go from an initial state through multiple intermediate steps to a final state rather than just from state A to state B
  • run forwards, in reverse, or alternate directions
  • loop infinitely

The required approach is:

  1. use @keyframes to define a reusable “template” set of animation states (or frames); then
  2. apply animation properties to an element we want to animate, including one or more @keyframes to be used.

Here’s how you do it:

@keyframes flash {
  0%    { opacity: 0; }
  20%   { opacity: 1; }
  80%   { opacity: 0; }
  100%  { opacity: 1; }
}

.animate-me {
  animation: flash 5s infinite;
}

Note that you can also opt to include just one state in your @keyframes rule, usually the initial state (written as either from or 0%) or final state (written as either to or 100%). You’d tend to do that for a two-state animation where the other “state” is in the element’s default styles, and you’d either be starting from the default styles (if your single @keyframes state is to) or finishing on them (if your single @keyframes state is from).

Should I use transition or animation?

As far as I can tell there’s no major performance benefit of one over the other, so that’s not an issue.

When the animation will be triggered by pseudo-class-based events like :hover or :focus and is simple i.e. based on just two states, transition feels like the right choice.

Beyond that, the choice gets a bit less binary and seems to come down to developer preference. But here are a couple of notes that might help in making a decision.

For elements that need to “animate in” on page load such as an alert, or when newly added to the DOM such as items in a to-do list, an animation with keyframes feels the better choice. This is because transition requires the presence of two CSS rules, leading to dedicated JavaScript to grab the element and apply a class, whereas animation requires only one and can move between initial and final states automatically including inserting a delay before starting.

For animations that involve many frames; control over the number of iterations; or looping… use @keyframes and animation.

For utility classes and classes that get added by JS to existing, visible elements following an event, either approach could be used. Arguably transition is the slightly simpler and more elegant CSS to write if it covers your needs. Then again, you might want to reuse the animations applied by those classes for both existing, visible elements and new, animated-in elements, in which case you might feel that instead using @keyframes and animation covers more situations.

Performance

A smooth animation should run at 60fps (frames per second). Animations that are too computationally expensive result in frames being dropped, i.e. a reduced fps rate, making the animation appear janky.

Cheap and slick properties

The CSS properties transform and opacity are very cheap to animate. Also, browsers often optimise these types of animation using hardware acceleration. To hint to the browser that it should optimise an animation property (and to ensure it is handled by the GPU rather than passed from CPU to GPU causing a noticeable glitch) we should use the CSS will-change property.

.my-element {
  will-change: transform;
}

Expensive properties

CSS properties which affect layout such as height are very expensive to animate. Animating height causes a chain reaction where sibling elements have to move too. Use transform over layout-affecting properties such as width or left if you can.

Some other CSS properties are less expensive but still not ideal, for example background-color. It doesn't affect layout but requires a repaint per frame.

Test your animations on a popular low-end device.

Timing functions

  • linear goes at the same rate from start to finish. It’s not like most motion in the real world.
  • ease-out starts fast then gets really slow. Good for things that come in from off-screen, like a modal dialogue.
  • ease-in starts slow then gets really fast. Good for moving somethng off-screen.
  • ease-in-out is the combination of the previous two. It‘s symmetrical, having an equal amount of acceleration and deceleration. Good for things that happen in a loop such as element fading in and out.
  • ease is the default value and features a brief ramp-up, then a lot of deceleration. It’s a good option for most general case motion that doesn’t enter or exit the viewport.

Practical examples

You can find lots of animation inspiration in libraries such as animate.css (and be sure to check animate.css on github where you can search their source for specific @keyframe animation styles).

But here are a few specific examples of animations I or teams I’ve worked on have had to implement.

Skip to content

The anchor’s State A sees its position fixed—i.e. positioned relative to the viewport—but then moved out of sight above it via transform: translateY(-10em). However its :focus styles define a State B where the intial translate has been undone so that the link is visible (transform: translateY(0em)). If we transition the transform property then we can animate the change of state over a chosen duration, and with our preferred timing function for the acceleration curve.

HTML:

<div class="u-visually-hidden-until-focused">
  <a
    href="#skip-link-target"
    class="u-visually-hidden-until-focused__item"
  >Skip to main content</a>
</div>

<nav>
  <ul>
    <li><a href="/">Home</a></li>
    <li><a href="/">News</a></li>
    <li><a href="/">About</a></li>
    <!-- …lots more nav links… -->
    <li><a href="/">Contact</a></li>
  </ul>
</nav>

<main id="skip-link-target">
  <h1>This is the Main content</h1>
  <p>Lorem ipsum <a href="/news/">dolor sit amet</a> consectetur adipisicing elit.</p>
  <p>Lorem ipsum dolor sit amet consectetur adipisicing elit.</p>
</main>

CSS:

.u-visually-hidden-until-focused {
  left: -100vw;
  position: absolute;

  &__item {
    position: fixed;
    top: 0;
    left: 0;
    transform: translateY(-10em);
    transition: transform 0.2s ease-in-out;

    &:focus {
      transform: translateY(0em);
    }
  }
}

To see this in action, visit my pen Hiding: visually hidden until focused and press the tab key.

Animating in an existing element

For this requirement we want an element to animate from invisible to visible on page load. I can imagine doing this with an image or an alert, for example. This is pretty straightforward with CSS only using @keyframes, opacity and animation.

Check out my fade in and out on page load with CSS codepen.

Animating in a newly added element

Stephanie Eckles shared a great CSS-only solution for animating in a newly added element which handily includes a Codepen demo. She mentions “CSS-only” because it’s common for developers to achieve the fancy animation via transition but that means needing to “make a fake event” via a JavaScript setTimeout() so that you can transition from the newly-added, invisible and class-free element state to adding a CSS class (perhaps called show) that contains the opacity:1, fancy transforms and a transition. However Stephanie’s alternative approach combines i) hiding the element in its default styles; with ii) an automatically-running animation that includes the necessary delay and also finishes in the keyframe’s single 100% state… to get the same effect minus the JavaScript.

Avoiding reliance on JS and finding a solution lower down the stack is always good.

HTML:

<button>Add List Item</button>
<ul>
  <li>Lorem ipsum dolor sit amet consectetur adipisicing elit. Nostrum facilis perspiciatis dignissimos, et dolores pariatur.</li>
</ul>

CSS:

li {
  animation: show 600ms 100ms cubic-bezier(0.38, 0.97, 0.56, 0.76) forwards;

  // Prestate
  opacity: 0;
  // remove transform for just a fade-in
  transform: rotateX(-90deg);
  transform-origin: top center;
}

@keyframes show {
  100% {
    opacity: 1;
    transform: none;
  }
}

Jhey Tompkins shared another CSS-only technique for adding elements to the DOM with snazzy entrance animations. He also uses just a single @keyframes state but in his case the from state which he uses to set the element’s initial opacity:0, then in his animation he uses an animation-fill-mode of both (rather than forwards as Stephanie used).

I can’t profess to fully understand both however if you change Jhey’s example to use forwards instead, then the element being animated in will temporarily appear before the animation starts (which ain’t good) rather than being initially invisible. Changing it to backwards gets us back on track, so I guess the necessary value relates to whether you’re going for from/0% or to/100%… and both just covers you for both cases. I’d probably try to use the appropriate one rather than both just in case there’s a performance implication.

Animated disclosure

Here’s an interesting conundrum.

For disclosure (i.e. collapse and expand) widgets, I tend to either use the native HTML <details> element if possible or else a simple, accessible DIY disclosure in which activating a trigger toggles a nearby content element’s visibility. In both cases, there’s no animation; the change from hidden to revealed and back again is immediate.

To my mind it’s generally preferable to keep it simple and avoid animating a disclosure widget. For a start, it’s tricky! The <details> element can’t be (easily) animated. And if using a DIY widget it’ll likely involve animating one of the expensive properties. Animating height or max-height is also gnarly when working with variable (auto) length content and often requires developers to go beyond CSS and reach for JavaScript to calculate computed element heights. Lastly, forgetting the technical challenges, there’s often no real need to animate disclosure; it might only hinder rather than help the user experience.

But let’s just say you have to do it, perhaps because the design spec requires it (like in BBC Sounds’ expanding and collapsing tracklists when viewed on narrow screens).

Options:

  • Animate the <details> element. This is a nice, standards-oriented approach. But it might only be viable for when you don’t need to mess with <details> appearance too much. We’d struggle to apply very custom styles, or to handle a “show the first few list items but not all” requirement like in the BBC Sounds example;
  • Animate CSS Grid. This is a nice idea but for now the animation only works in Firefox*. It’d be great to just consider it a progressive enhancement so it just depends on whether the animation is deemed core to the experience;
  • Animate from a max-height of 0 to “something sufficient” (my pen is inspired by Scott O’Hara’s disclosure example). This is workable but not ideal; you kinda need to set a max-height sweetspot otherwise your animation will be delayed and too long. You could of course add some JavaScript to get the exact necessary height then set it. BBC use max-height for their tracklist animation and those tracklists likely vary in length so I expect they use some JavaScript for height calculation.

* Update 20/2/23: the “animate CSS Grid” option now has wide browser support and is probably my preferred approach. I made a codepen that demonstrates a disclosure widget with animation of grid-template-rows.

Ringing bell icon

To be written.

Pulsing “radar” effect

To be written.

Accessibility

Accessibility and animation can co-exist, as Cassie Evans explains in her CSS-Tricks article Empathetic Animation. We should consider which parts of our website are suited to animation (for example perhaps not on serious, time-sensitive tasks) and we can also respect reduced motion preferences at a global level or in a more finer-grained way per component.

Notes

  • transition-delay can be useful for avoiding common annoyances, such as when a dropdown menu that appears on hover disappears when you try to move the cursor to it.

References

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