"It doesn't trigger built-in mobile operating system components." Is that part of the spec? I can see Apple deviating from that implementation.
I'm not sure about this. It could break the positioning / sizing / box CSS properties.
That would require rendering arbitrary HTML in the native widget, outside the browser. And I think that would require putting WebKit in the native widget. Or, to maintain a "copy" of the widget that looks like the native one but uses WebKit to render things. Seems annoying to maintain.
(And there are the security concerns mentioned by the other comments)