Todo List

Core:
 - Finish all the Core CSS traits
 - Implement all non-essential attribute transforms

Code issues:
 - Massive profiling (library occasionally times out on large input)
 - Fix IPv6 issues
 - Make URI validation routines tighter (especially mailto)
 - Distinguish between different types of URIs, for instance, a mailto URI
   in IMG SRC is nonsensical
 - Factor out Host validation to its own AttrDef
 - Rewrite table's child definition

Enhancements:
 - Do fixes for Firefox's inability to handle COL alignment props (Bug 915)