Opera had the "draconical" approach, where upon the error you just had that, an error. Firefox, iirc had a softer approach where you still got the page rendered

Opera had the "draconical" approach, where upon the error you just had that, an error. Firefox, iirc had a softer approach where you still got the page rendered, but you'd get the error reported too. Anyway it all depended on the proper MIME type for the XTHML (as it should). However the whole MIME type and everything associated with it (some elements and APIs are treated differently) is a whole barrel of worms, so XHTML in any of the incarnations was never a good idea.