On Wed, 8 Dec 2004, Jan Egil Kristiansen wrote:

> I'd like a DOCTYPE to make it possible to validate XHTML with
> well-formed extensions from any other namespace.

A document type definition (which is what you refer to when you use a
DOCTYPE declaration) defines a specific document syntax, or document type
in the syntactic sense. It would be against the very idea of a DTD to
permit arbitrary elements.

So you can define a DTD type you like, but it needs to be specific, and it
won't be (X)HTML any more, except in a tag soup sense (you can use tags
that also appear in (X)HTML, and you could mentally assign similar
meanings to them).

> See http://styrheim.weblogg.no/081204103845_the_x_in_xhtml.html 

There you write: "The X in XHTML means Extensible. And in the browsers, it
really is. If I add an element of my own, maybe mine:something, an HTML
browser will ignore the mine:something tags, and display the
mine:something content, if any."
That's a common misunderstanding. XHTML is no more extensible than classic
HTML. Browsers have always ignored tags and attributes they don't
recognize, and for XHTML this is more questionable than for classic HTML
(for which the specs more or less recommend such processing).

-- 
Jukka "Yucca" Korpela, http://www.cs.tut.fi/~jkorpela/