IIRC several major browsers implemented XHTML error handling, but only for documents with a Content-Type: application/xhtml+xml header, which was basically nothing because that would then trip up other browsers
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.
IIRC Opera implemented XHTML error handling.