It's amazing how fast things can happen. I wrote two days ago that we are looking for good bug reports about HTML5 video support in Chromium/Gentoo. Soon after that, we got bug #314977, and after comparing a working and non-working configuration, and some testing, the fix was made available in the portage tree.
Today, after even more encouraging reports in the forums, I applied the fix to all www-client/chromium ebuilds in the tree.
The one thing I'd like to say now is... thank you, Gentoo community!