[anjbe]

> All browsers implement XHTML.

Internet Explorer did not. It completely refused to render XHTML pages served with an XML MIME type (application/xhtml+xml). It would only display pages if they were served with the text/html MIME type, which meant that none of XML’s vaunted features (such as strict parsing) came into play, and such pages were effectively treated as “HTML with syntax errors.”

A big part of why WHATWG was able to dethrone W3C was W3C’s insistence on dropping HTML in favor of XHTML when the overwhelmingly dominant browser of the time had zero support for it.

> They came up with XML, the clean subset, and XHTML, the DTD. … Basically, XHTML was the first actual standardization of HTML.

No, the first formal HTML standard was 2.0 (RFC 1866), which was released in November 1995 and had a DTD that among other things disallowed overlapping hierarchies. XML’s first draft was released a full year later (November 1996), and the first W3C spec was XML 1.0 in 1998. Later that year came the initial drafts for XHTML 1.0, which was a straightforward translation of HTML 4.0 to XML.