[css-highlight-api][css-pseudo] add style attribute to pseudo-elements and use it in css-highlight-api as a replacement for the style attribute on Highlight

[frivoal]

Exploratory Pull Request drafting what I suspect would be a good solution to #4588 (and to #4602 as well).

[sanketj]

The CSSPseudoElement interface currently only supports ::before, ::after and ::marker (lines 876-879). These are all tree-abiding pseudo elements, meaning that they have a well-defined tree relationship with their owning element, and as a result, we can craft a sensible event flow when one of these receive an event.

CSSPseudoElement does not support (at least yet!) any pseudo elements that inherit styles from other pseudo elements, like ::selection or ::highlight, and not even pseudos that apply styles to multiple elements, like ::first-line. These seem harder to support because it's not clear what the event flow should be and how we would implement it (related issue: #4398 ). If we keep the same event path as we do currently, we probably won't deliver events to non-owning DOM elements that are covered by the pseudo. We would need to define a new event path and make sure it also works for other types of events (like pointer events - these are important for highlight events).

I'm happy to brainstorm about this further (assuming people want to expand CSSPseudoElement to non-tree abiding elements). If either you and @fantasai have any ideas or know of any discussions that have taken place about this before, I would love to hear them. I'll also ponder on this and try to bring something to discuss during our next highlight API discussion call.

[frivoal]

Closed #4588 and #4602 differently. See 78f022f