There was XHTML and HTML5 is a direct result of finding out that was not the right solution. The main issue that was being solved there was that browsers do not

There was XHTML and HTML5 is a direct result of finding out that was not the right solution. The main issue that was being solved there was that browsers do not parse invalid plain HTML consistently, which XHTML solved by requiring invalid XHTML to be rejected outright. This did not work. HTML5 solves this by defining the parsing rules such that there is a concept of document being invalid, every sequence of bytes deterministically maps to one particular DOM tree. This feature essentially precluded basing HTML5 on either XML (simply impossible) or SGML (that might be possible, but is in fact redundant formalism and describing the syntax in prose makes more sense, as everybody is going to hand-craft the parser anyway).