Add additional hooks for callers:

* Resolve relative URLs.
* Set priority on any stylesheet implementation.
* Substitute content during XML Conduit traversal in place of computed styles.

Rework psuedoelement infrastructure so they can be their own boxes.

Code cleanliness fixes.

Parse HTML 'style' attributes.

Fix parser freezes, blocking issue for Rhapsode.

Fix incorrect handling of invalid syntaxes.

Ignore invalid selectors.

Switch to new namespace for consistency with other modules like css-syntax.

Draft alternate API.