Define which elements can be which kinds of widgets for CSS 'appearance'

Closes #7004 by superseding it. See also w3c/csswg-drafts#6537 and w3c/csswg-drafts#7224.

Co-authored-by: fantasai <fantasai.bugs@inkedblade.net>
Co-authored-by: Simon Pieters <zcorpan@gmail.com>
Co-authored-by: Howard Edwards <howarde.edwards@gmail.com>

Author: 4 people