Use "user attention" of top-level traversable

[marcoscaceres]

See: https://github.com/whatwg/html/commit/2db564c6902ace8c2539560e508ce22c2948c4d8

Handles a bunch of stuff we do in the spec "for free"™️.

The definition to ref is [=Document/fully active descendant of a top-level traversible with user attention=]