Confirmed on Debian and Fedora, with WebKit 1.8.x and Midori, Epiphany and GtkLauncher, YouTube’s HTML5 player does not work anymore and just a black box is shown. WebKit 1.10.1 is supposed to work according to a user in #midori on irc.freenode.net.
Sorry I didn't see this bug... does it work now with webkit2gtk 2.4.x ? It works for me, please reopen if the issue is still present.