[cssom] Replace "browsing context container" link

[tabatkins]

Currently, the definition of what styles are exposed on pseudo-elements in getComputedStyle() attempt to link to the term "browsing context container". This link fails, tho, because the term is intentionally not exported by its defining spec (https://w3c.github.io/longtasks/#browsing-context-container), with the note "This term is outdated, and the new terms should be reused when revamping this."

(This text was commited seven years ago by @emilio in 00c78bc, when defining that detached/etc elements don't expose styles.)

I'm not sure what terms to replace this with. @emilio ?

[emilio]

(I think the first link is wrong and should point to https://drafts.csswg.org/cssom-1/#dom-window-getcomputedstyle right?).

I think the idea would be to use https://html.spec.whatwg.org/#node-navigable, but there are some hoops to define what we want (the document is not renderer).

Something like doc's node navigable's active browsing context's embedder element is not being rendered, or something? But there's no link from navigable to its embedder, afaict... @annevk am I missing something? Should there be?

[annevk]

I think it should be something like:

... and its shadow-including root's node navigable's container is null or is being rendered, then set ...

Happy to review a PR and I'm sure @domenic would be as well.

(See https://html.spec.whatwg.org/#nav-container for the definition of container.)

[emilio]

Ah, so the navigable container is the <iframe> element or null, that is exactly what we want then, thanks Anne!