NEM - XHTML is the newest version of HTML from the W3C[L1 ]. It's an application of XML, whereas HTML is an application of SGML. XHTML is moving towards semantic markup. The idea being that the XHTML contains only markup which describes the structure of the document. Any presentation is then handled by CSS stylesheets or something similar. It's a nice idea in theory, but we'll have to wait to see how it turns out in practice (most pages still use HTML, and particularly, use tables for layout). Being XML, Tcl can parse XHTML using either tDOM or TclXML. I'm not aware of any CSS parser for Tcl.
There are a bunch of new technologies being developed alongside XHTML, and which will probably end up being used with it - e.g. XForms, MathML (and OpenMath), XLink, XQuery etc.