Would Google index attributes in XLink namespace? E.g.:

    <img src="pic.jpg" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="about.html" />

I know they index SVG documents, and there’s no @href in SVG namespace… Would that work for (X)HTML5 documents, either served as HTML or XML document?

You’d still need JS polyfill, but at least the semantics remain intact.