We need to explain the difference between XHTML and HTML somewhere, probably earlier in the introduction in fact, but the problem is I don't know how to explain it. If someone can come up with some good text to describe the difference and similarities between the two (namely that they are just different serialisations of the same in-memory structure, with a few caveats), then please send it to the WHATWG list so I can include it. The problem is making it accurate while keeping it simple enough that people who don't know what a serialisation of an in-memory structure is can still get it. (The relevant section doesn't have to be normative, if that helps.) The sentence "Every XML and HTML document in an HTML UA is represented by a Document object." from section 2.3 should seem perfectly reasonable after someone has read this section.