Ummm, that already exists. That's how XHTML (delivered as XML) works. Make a syntax error in the page? Browser gives up, displays an error. It didn't catch on.

Ummm, that already exists. That's how XHTML (delivered as XML) works. Make a syntax error in the page? Browser gives up, displays an error. It didn't catch on. The various HTML strict modes turn off "quirks" mode as well.