And specifically XHTML failed for two reasons: (0) Internet Explorer did not support it, and (1) there was no good migration path out of tooling that generated

And specifically XHTML failed for two reasons: (0) Internet Explorer did not support it, and (1) there was no good migration path out of tooling that generated HTML by unrigorous string concatenation. So instead WHATWG created HTML5, which bastardized well-designed (if clunky to use) XML features like namespaces, paving the way for vulnerabilities like CVE-2020-26870.