After more research, it looks like this is a browser support issue. See:
- https://code.google.com/p/chromium/issues/detail?id=109212
- https://bugs.webkit.org/show_bug.cgi?id=105904
Sadly, I’d come across this question before posting mine, and had thought that surely, in 5-1/2 years, browser support would have caught up – but that doesn’t appear to be the case.
As of 2015, apparently Firefox and Opera are the only two browsers to support this in any substantial way.
Back to the drawing board…