Now we only look for hints that are actually possible for the
given element. This new way of doing things is made possible
by the new LibCSS API for hints.
This also makes use of the LibDOM HTMLElement tag type to avoid
lots of massive if-else-if ladders of dom_string_caseless_isequal
calls.
Collect hints and provide them up front.
Note this implementation is minimal effort, and quite inefficient.
We can be faster about it by walking the element's attributes, and
caching things set on ancestor elements which apply to the current
element. (Mostly table, and body stuff.)
