[keeperofdakeys]

One of the big differences between HTML4 and HTML5 is that implicit closing tags are defined in the spec, and not just a consequence of browser implementations. So "error handling" in HTML4 has essentially become a feature in HTML5

For XHTML, one of the big ideas was that you could use an XML parser, and embed custom XML. Since an XML parser errors on invalid input, it can be smaller and faster. Having an XML parser also means embedded XML is easy to deal with. However, all this falls down when you consider that nearly all XHTML was sent as HTML, so the XML parser never kicked in. All this meant you required properly formatted files.

[mindcrime]

Sadly, Microsoft deserve a fair amount of blame for this, for not ever really supporting XHTML in IE back when it was so dominant. Oh, I mean, they "supported" it in that it would render, but they didn't support the application/xhtml+xml content-type, which mean that, in turn, nobody served their XHTML as application/xhtml+xml, and so on.

I won't say the lack of widespread adoption of XHTML was all Microsoft's fault, but they definitely played a role.