I entered the following into my .xul file: <html:h1>Hello World!!!<blah></h1></blah> (hope you get that. It does broken namespaces and crossed over tags.) And it parses it just fine with no errors and produces a h1 header. You don't want to make the same mistakes as IE5 - I'm not sure why this isn't being picked up - I thought you were using expat which I know picks up the error - I tried it in perl's XML::Parser. Matt.

This would definitely get an error in expat. We just haven't enabled expat by default yet. I'll close this bug once the defalt usage of expat comes online early next week.

Setting milestone to M4...

Expat is enabled by default. The above XUL is going to throw a well formedness error. Marking fixed.