The `:scope` pseudo-class[1] has surprisingly good browser support: Chrome,
Firefox & Safari have supported if for a long time; only IE & Edge lack support.
This commit leverages this pseudo-class to get rid of the ID hack in most cases.
Adding a temporary ID may cause layout thrashing which was reported a few times
in [the past.
We can't completely eliminate the ID hack in modern browses as sibling selectors
require us to change context to the parent and then `:scope` stops applying to
what we'd like. But it'd still improve performance in the vast majority of
cases.
[1] https://developer.mozilla.org/en-US/docs/Web/CSS/:scope
Fixes gh-4453
Closes gh-4454
Ref gh-4332
Ref jquery/sizzle#405
df6a7f7f0f