Tangent: the fact XHTML didn't gain traction is a mistake we've been paying off for decades.

Browser engines could've been simpler; web development tools could've been more robust and powerful much earlier; we would be able to rely on XSLT and invent other ways of processing and consuming web content; we would have proper XHTML modules, instead of the half-baked Web Components we have today. Etc.

Instead, we got standards built on poorly specified conventions, and we still have to rely on 3rd-party frameworks to build anything beyond a toy web site.

Stricter web documents wouldn't have fixed all our problems, but they would have certainly made a big impact for the better.