More than words can say

"There has never been a better time, in the history of the planet, to pick up a physical skill on a 'teach yourself' basis."-- More than words can say

HTML v. XHTML

Bill quotes Sam:

At the moment, HTML and XHTML are two different authoring formats covering roughly the same domain of DOM trees.

By ignoring trivia such as actual history, an alternative view is:

At the moment, HTML is a Domain Specific language (DSL) for authoring XHTML.

What is "domain specific" about HTML that is not equally domain specific about XHTML you might be thinking? In a word: authoring.
In the transition from SGML to XML a whole bunch of authoring conveniences were dropped. The thought was that tech advances (GUIs etc) made them unnecessary.
History would suggest otherwise I think.