XML isn't "this sort of thing with tags and attributes". It's a rigidly-specified format, and for any given series of bytes and an encoding, it either is or is not XML. If there's a "bug in his XML", it isn't XML anymore.

You can talk about what to do with this not-XML, but you can't just pretend it is XML. It isn't, because XML is not just this fuzzy thing, it's a very, very, very specific thing.