If browsers would error out on invalid content then those services would either provide valid content or they would go bust.

And XML/XHTML is much easier to parse and for producing valid content than HTML because it is much more consistent and has less history package than HTML.

It is just that today, we still have tag-soup and error-tolerant parsers in the browsers and of course lots and lots HTML producer are producing shitty HTML and you can't just switch on a strict parsing.

But if history would have taken a slightly different turn, we would be talking about XHTML5 and not HTML5.