This is required for `document.execCommand`, since it needs to check the
active range of the selection whether it has the attribute or not.
It also turns on the relevant WPT tests, with 1
pre-existing failure for absolute positioned pseudo elements. These
currently do not render correctly, regardless of the existence of
contenteditable.
Fixes#12776
Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>
