XHTML is dead.
This is not due to a failure to implement the standard but due to the fact the standard was misdesigned.
See that XHTML's 1.0's claimed HTML 4.01 compatability was based on pure fiction as <br/> would actually place an angle bracket on the output page if parsed by a strict HTML 4.01 parser.
But that is not what killed XHTML.
XHTML is dead because HTML 5 is inherited directly from HTML 4.01 and does not use an XML parser.
Another issue: <script> print a & b </script>.
Due to the generation of kiddie programmers raised belting out thousands of lines of HTML in Notepad, a web browser does not have the luxury of expecting all scripted ampersands & to be escaped &. Hence, ampersands must NOT be escaped, because the browser must not misinterpret them and double-escape them.
Hence, HTML and XML remain sibling languages, never to merge.