Make the serialiser match the HTML fragment serialization algorithm

[gsnedders]

Linkity link.

A lot of this is essentially is essentially changing defaults for the serialiser. The only particularly questionable part is the fact it never serialises PUBLIC/SYSTEM parts of DOCTYPEs, so we probably need a new option for that.

[willkg]

There hasn't been any progress on this that I can see, so I'm bumping it out of the 1.0 milestone.