HTML is parsed by SGML, which is a very loose way of doing things and will let you get away with pretty much anything. XHTML, like the article says is more strict. But XHTML should not be parsed by SGML, it should be parsed by XML. No problem there, untill you send it to IE and BAM! IE wants to download the page and do all stupid things with it (there goes ~90% of browser support).

Plus there is no advantage with XHTML over HTML.