Thought experiment [dive into mark]
Norman Walsh (invalid XML), Danny Ayers (invalid XML), Brent Simmons (invalid XML), Nick Bradbury (invalid XML), and Joe Gregorio (invalid XML claiming to be HTML) have all denounced me as a heretic for pointing out that, perhaps, rejecting invalid XML on the client side is a bad idea. The reason I know that they have denounced me is that I read what they had to say, and the reason I was able to read what they had to say is that my browser is very forgiving of all their various XML wellformedness and validity errors.
Tim Bray has chimed in by calling all of those people names, stating in no uncertain terms that anyone who can’t make … well-formed XML is an incompetent fool. Well, technically he was only talking about syndication feeds, but since XHTML is as much XML as Atom or RSS, I’m pretty sure he would apply the same measurement. So if you can’t make well-formed XML, don’t despair; you may be a fool, but you are, if nothing else, in outstanding company.
Imagine, if you will, that all web browsers use strict XML parsers. That is, whenever they encounter an XHTML page, they parse it with a conforming XML parser and refuse to display the page unless it is well-formed XML. This part of the thought experiment is not terribly difficult to imagine, since Mozilla actually works this way under certain circumstances (if the server sends an XHTML page with the MIME type application/xhtml+xml, instead of the normal text/html). But imagine that all browsers worked this way, regardless of MIME type.
