It is unclear which Document object https://drafts.csswg.org/resize-observer/#ref-for-resizeobserver is talking about

[smaug----]

"Add this to Document.[[resizeObservers]] slot."

What is that Document? I assume the idea is to get that from a global, but which global?

[emilio]

Gecko at least uses the current global's document at the point of construction of the observer.

I think that's also what WebKit does given the CurrentDocument here.

Not quite sure what Blink does / what the ScriptState corresponds to there... @chrishtr do you know?

In any case, it seems current on construction is probably the most straight-forward answer here.

This is observable via callback order, cc @Loirooriol

[chrishtr]

Blink uses current global's document also.