HTML as common glue

Why HTML? None of the browsers yet supports a non-HTML way of displaying images. Microsoft doesn't support (and hasn't announced plans to support) XLink, and doesn't yet support Cascading Style Sheets Level 2 for table construction. To get these basic features, developers need to fall back on HTML. (Scripting XML with ECMAScript/JavaScript remains out of the question until Opera catches up.)
Netscape and Opera use namespace URIs Both Netscape and Opera recognize HTML elements in the namespace "http://www.w3.org/TR/REC-html40". (To conform with XHTML, it should be ""http//www.w3.org/1999/xhtml"", but Opera doesn't yet support that.) This means that the crucial a and img elements are available for Web page layout, along with HTML tables.
Microsoft uses fixed html: prefix Microsoft will let you use any URI you like, provided that the prefix used on the elements is html:. As a result, declaring xmlns:html="http://www.w3.org/TR/REC-html40" allows documents which use both XML and HTML.
Moving toward an XHTML base? XHTML is the W3C's effort to place the HTML vocabulary on an XML foundation. It's far more formal than the ad hoc approach outlined above, but it could conceivably build on this approach.