So, you’d like to animate the display property

Die CSS Working Group gab dem vor ein paar Wochen grünes Licht. Der super-duper konzeptionelle Vorschlag ist, dass wir von z.B. display: block zu display: none animieren oder überleiten können.

It’s a bit of a brain-twister to reason about because setting display: none on an element cancels animations. And adding it restarts animations. Per the spec

Setting the display property to none will terminate any running animation applied to the element and its descendants. If an element has a display of none, updating display to a value other than none will start all animations applied to the element by the animation-name property, as well as all animations applied to descendants with display other than none.

That circular behavior is what makes the concept seemingly dead on arrival. But if @keyframes supported any display value other than none, then there’s no way for none to cancel or restart things. That gives non-none values priority, allowing none to do its thing only after the animation or transition has completed.

Miriam’s toot (this is what we’re really calling these, right?) explains how this might work

We’re not exactly interpolating between, say, block and none, but allowing block to stay intact until the time things stop moving and it’s safe to apply none. These are keywords, so there are no explicit values between the two. As such, this remains a discrete animation. We’re toggling between two values once that animation is complete.

This is the Robert Flack’s example pulled straight from the discussion

@keyframes slideaway {
  from { display: block; }
  to { transform: translateY(40px); opacity: 0;}
}

.hide {
  animation: slideaway 200ms;
  display: none;
}

This is a helpful example because it shows how the first frame sets the element to display: block, which is given priority over the underlying display: none as a non-none value. That allows the animation to run and finish without none cancelling or resetting it in the process since it only resolves after the animation.

This is the example Miriam referenced on Mastodon

.hide {
  transition: opacity 200ms, display 200ms;
  display: none;
  opacity: 0;
}

We’re dealing with a transition this time. The underlying display value is set to none before anything happens, so it’s completely out of the document flow. Now, if we were to transition this on hover, maybe like this

.hide:hover {
  display: block;
  opacity: 1;
}

…then the element should theoretically fade in at 200ms. Again, we’re toggling between display values, but block is given priority so the transition isn’t cancelled up front and is actually applied after opacity finishes its transition.

At least that’s how my mind is reading into it. I’m glad there are super smart people thinking these things through because I imagine there’s a ton to sort out. Like, what happens if multiple animations are assigned to an element — will none reset or cancel any of those? I’m sure everything from infinite animations, reversed directions, and all sorts of other things will be addressed in time.

But what a super cool first step!