[othermaciej]

We've tried this experiment - it's called XHTML. For a long time, adoption by authors of the strict error handling it offered was stymied by lack of support in MSIE. So it's not a full counter-factual. However, we have learned two things:

(1) Now that MSIE does support true XML parsing of XHTML, almost no one is choosing to use it over HTML.

(2) Of the few experts who conditionally served either text/html or application/xhtml+xml depending on the UA, or serve XML unconditionally now, almost all have bugs in their sites which can get them to produce ill-formed XML which then shows an error page in the browser (for instance, submitting comments with certain sorts of errors). This is evidence that the draconian error handling approach is too challenging even for experts and imposes the costs of small mistakes on users.

[tedunangst]

I think the bigger lesson to be learned is that after poorly followed ad hoc standards have made a mess of things, it's hard to come in and clean up later.