[css-view-transitions-2] [css-pseudo-4] Clarify ordering of ::view-transition with other tree-abiding pseudo-elements

[khushalsagar]

CSS Pseudo spec defines tree-abiding pseudo-elements which are pseudo-elements that fit in the box tree like DOM elements. All of the view-transition pseudo-elements are also tree-abiding pseudo-elements. While the View Transition spec has a well defined structure (and DOM order) of the internal pseudo-elements, we need to explicitly define the DOM order of ::view-transition with respect to other pseudo-elements.

Proposed Resolution: ::view-transition is the last child of its originating element, i.e., following ::after.

I don't think the above matters during layout/paint. ::view-transition has its own special containing block and a stacking context (which paints last). Style could've been affected by it if selectors like nth-child applied but they don't apply to pseudo-elements. So my rationale for the above resolution is that it being last conceptually fits the fact that its painted last.

Side-note: The DOM order for existing tree-abiding pseudo-elements should probably be spec'd in css-pseudo. I only found it in the animation spec here. But we should ideally define it in the pseudo spec which is referenced by the animation spec.

@flackr @bokand FYI. @fantasai @astearns because this touches the pseudo spec.

[bokand]

Thanks @khushalsagar - one more thing to consider: would it make sense to specify an ordering for the descendant pseudos of ::view-transition? At least for the Animation composite order case? It currently states:

any other pseudo-elements not mentioned specifically in this list, sorted in ascending order by the Unicode codepoints that make up each selector

We actually have an explicit paint ordering for the view-transition-names so I think it'd make sense to order ::view-transition-group based on that rather than lexographically. Similarly for ::view-transition-new and ::view-transition-old.

[khushalsagar]

We actually have an explicit paint ordering for the view-transition-names so I think it'd make sense to order ::view-transition-group based on that rather than lexographically.

That's how its in the spec today, see the text here. We actually rely on the DOM order of the group pseudos to make sure their paint order in the pseudo-DOM matches their paint order in the author DOM.