[css-contain-2] Stop monkey-patching the HTML spec.

[emilio]

This exports an "update content relevancy for a document" algorithm that HTML can call at the right time.

[vmpstr]

The new text looks good to me. One unfortunate consequence is that we always run the style&layout steps even if we know we have pending relevance changes which as far as I can tell pretty much guarantee that we'll have to run style&layout again.

Perhaps this is in the realm of browsers being able to optimize this, but it's unclear if there's interactions with other features that do require the initial style&layout before relevant state changes

[emilio]

The new text looks good to me. One unfortunate consequence is that we always run the style&layout steps even if we know we have pending relevance changes which as far as I can tell pretty much guarantee that we'll have to run style&layout again.

I think that's necessary tho, right? Otherwise whether you see the effect of something like:

el.focus();
el.style.contentVisibility = "auto";

On this frame or the next depends on whether something has updated style and layout in the interim, right?