mattur - 9:36 pm on Nov 3, 2008 (gmt 0)

I'm wondering how a HTML5-compliant non-visual UA would be supposed to deal with an <i> element in an old document (before HTML5 it should normally be ignored, because it is only presentational, but now HTML5 gives it a meaning...)

And that meaning is: "a span of text in an alternate voice or mood... whose typical typographic presentation is italicized"

I suspect this isn't going to break the web :)

...picking single XHTML2 features and make them work on older [well-formed XHTML] documents is also achievable, and exactly by the same means.

Emphasis added. But what about namespace clashes eg <input> in XForms and XHTML1 Forms?

XML had the "fail on error" issue as a cost... but it bought something with that cost: extensibility

The problem for XHTML is that extensibility doesn't require "fail on error" as a cost...