> But who cares? The browser isn't validating HTML as it's rendering it, otherwise the internet would be one big blank page.

But that's the big difference isn't it? Browsers fix html as they go, but they blow up on XML (as mandated by the XML spec).

What would be weirder, seeing the page either render fully or blow up, or seeing a page start rendering and then at some point be replaced by an error message?

Of course ideally it would start rendering and keep rendering, never blowing up, but that's not really allowed in XML.