Presentation attribute lengths (width, height, etc.) can always be
unit-less (e.g "400") so going via the normal CSS parsing path only
works when the document is in quirks mode.
Add a separate parse_html_length() that always allows unit-less values.
"width: 500" is not a valid CSS property in standards mode and should
be ignored.
To plumb the quirks-mode flag into CSS parsing, this patch adds a new
CSS::ParsingContext object that must be passed to the CSS parser.
Currently it only allows you to check the quirks-mode flag. In the
future it will be a good place to put additional information needed
for things like relative URL resolution, etc.
This narrows <div class=parser> on ACID2 to the correct width. :^)
