> as XML had much more well-defined implementation requirements, this among other things meant that web pages that weren't well-formed XML would have to break b

> as XML had much more well-defined implementation requirements, this among other things meant that web pages that weren't well-formed XML would have to break by definition and browsers were only allowed to show a yellow page of death with the syntax error. Indeed, part of the problem with switching to XHTML is that, even today, much HTML is authored by hand. On top of that web pages are often a mix of content from various sources. Third party widgets (like Twitter or ads) might be injected into the page. Or user content, which has often been converted by a hodgepodge of complex regexes into something vaguely resembling HTML.