Re: [csswg-drafts] [css-view-transitions-1] Define the constraints which must be satisfied by a named element during the transition (#8548)

Summary for CSSWG meeting:

[Previous resolution](https://github.com/w3c/csswg-drafts/issues/8139): A `view-transition-name` value that isn't `none` gives an element stacking context, grouping element, backdrop root, similar to non-1 `opacity`, as view transitions need these constraints.

The value of `view-transition-name` is only consulted during the setup of the view transition, to decide which elements should be involved in the transition. However, because the `::view-transition-new` view is 'live', the constraints need to be checked per frame.

[Previous resolution](https://github.com/w3c/csswg-drafts/issues/7882): If constraints are broken mid-transition, the rest of the transition is skipped.

There are additional constraints per-frame that aren't covered by `view-transition-name`, such as the element being rendered. These aren't constraints during the setup of the transition, as these constraints simply result in the element being ignored.

The question is, how should we check these conditions are met per frame?

**Option 1**: Assert the additional conditions, such as "rendered", then assert the remaining conditions by checking the `view-transition-name` is not none.

This means removing the `view-transition-name` from a transitioning element mid-transition would skip the transition. However, setting the `view-transition-name` to another value has no effect - it wouldn't change the transition group of the element. We're only concerned with the side-effects of `view-transition-name`.

**Option 2**: Assert all of the conditions individually, regardless of `view-transition-name`.

This means removing the `view-transition-name` from a transitioning element mid-transition would skip the transition, unless the constraints are satisfied by other means, such as a non-0 `opacity`.

We're split on this, so help us decide 😄 

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Received on Wednesday, 15 March 2023 11:07:09 UTC