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[csswg-drafts] [web-animations-1] Clarification about `target` and `pseudoElement` relationship (#4745)

From: Antoine Quint via GitHub <sysbot+gh@w3.org>
Date: Wed, 05 Feb 2020 17:22:25 +0000
To: public-css-archive@w3.org
Message-ID: <issues.opened-560517948-1580923343-sysbot+gh@w3.org>
graouts has just created a new issue for https://github.com/w3c/csswg-drafts:

== [web-animations-1] Clarification about `target` and `pseudoElement` relationship ==
I'm not sure the spec is clear about what the animation target is if `pseudoElement` is not `null`.

Consider these two tests in web-animations/interfaces/Animatable/animate.html for instance:

```javascript
test(t => {
  const div = createDiv(t);
  div.classList.add('pseudo');
  getComputedStyle(div,"::before").content; // Sync style
  const anim = div.animate(null, {pseudoElement: '::before'});
  assert_equals(anim.effect.target, div, 'The returned element has the correct target element');
  assert_equals(anim.effect.pseudoElement, '::before',
                'The returned Animation targets the correct selector');
}, 'animate() with pseudoElement an Animation object targeting ' +
   'the correct pseudo-element');

test(t => {
  const div = createDiv(t);
  const anim = div.animate(null, {pseudoElement: '::before'});
  assert_equals(anim.effect.target, div, 'The returned element has the correct target element');
  assert_equals(anim.effect.pseudoElement, '::before',
                'The returned Animation targets the correct selector');
}, 'animate() with pseudoElement without content creates an Animation object targeting ' +
   'the correct pseudo-element');
```

In the first example, I understand what's happening: the target element has a `::before` pseudo-element with content and it's created *before* `Element.animate()` is called. It seems clear that the animation target is the `::before` pseudo-element.

In the second example, I'm not so sure. When `Element.animate()` is called the target `div` has no `::before` pseudo-element. So what is the effective target of that animation? Does it actually target any element? What if the animation starts and the `div` gets a `::before` pseudo-element? Does the animation then affects that element?

I guess my question is whether we need a targeted pseudo-element to exist when the API tries to reference it, or if it's all resolved live.

Similarly, I'm confused by this test in web-animations/interfaces/Animation/commitStyles.html:

```javascript
test(t => {
  const div = createDiv(t);
  div.classList.add('pseudo');
  const animation = div.animate(
    { opacity: 0 },
    { duration: 1, fill: 'forwards', pseudoElement: '::before' }
  );

  assert_throws_dom('NoModificationAllowedError', () => {
    animation.commitStyles();
  });
}, 'Throws if the target element is a pseudo element');
```

At the time `Animation.animate()` is called, style resolution has not happened yet and there is no `::before` pseudo-element for `div`. However, there would be when `Animation.commitStyles()` is called.

Up until `pseudoElement` was added, it was clear whether an animation was targeting something, because you'd have to pass a value to `KeyframeEffect.target`. Now that the target is specified across a pair of properties without a direct reference to an `Element`, it's not as clear.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/4745 using your GitHub account
Received on Wednesday, 5 February 2020 17:22:28 UTC

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