I believe that after reading this thread, I'd have to agree with Dave. The WC3 can't say what HTML is, only what "standard" HTML is. What about the <marquee> tag, or the <blink> tag? These both are valid html tags, just not standard tags. IE recognizes the <marquee> tag and NN the <blink>. If any one of us were to make a browser, we could add any tags that we want. It wouldn't be a good browser by any means, becuase it isn't standard, but the tags would still all be HTML. I personally thank God for standards. From a developers point of view, if every browser had their own tags, you would need a version of your page for every browser. But, I do agree with Dave in that these tags are valid HTML, just not standard.
[edited a long time later, when I found this thread in a search]
Just so no one holds any of the crap I wrote above (and below) against me, I wrote that when I was very unknowledgable about HTML and how the web worked. Jeff Mott and Stefan argued the correct point, and none of what I said in this thread is worth reading...