When using HtmlHelper for style deduplication, slight differences in
the serialization format used by the legacy parser caused test failures.
Add a "compatibility" mode which tries to better match legacy parser
behavior for void elements, character escapes, and other details.
Parsoid HTML has always been serialized using an HTML5 serializer,
so this compatibility mode will be disabled when processing Parsoid
HTML.
Change-Id: I0441aa3e44f6562e05e95a18cc282c53fe446788
