Honestly, I think the push to constantly add browser features (and thereby expand the already massive web API) is misguided. This feature, in particular, seems like it should be solved with library, rather than by further bloating the browser.

If I put on my conspiracy hat, it looks like Google is pushing to expand the web API as a means to maintain its lead in the browser market by making it harder for other browsers (i.e. Firefox) to maintain feature parity.