I'm in the same situation.
Since i've installed safari 5 i got no html5, neither apple.com videos nor youtube.com.
I have to say that some days ago i have the same issue with safari 4.0.5, but i could fix it installing 10.6.3 combo.
Today i have tried the same thing, i've reinstalled that combo, i've also repaired disk permissions, but i can't fix this issue ....
mine was even worse! not even the Apple website would show up; it was just blank white! played around with all kinds of suggestions here; nothing worked;
i just discovered that under the Develop drop down "Disable Images" was checked; i never saw it checked before; Safari 5 must have downloaded with it;
check to see if yours is checked and uncheck it; that may help...
Hey, Exact same problem, a black box instead of youtube videos,
Macbook Pro Intel Snow leopard.
The user agent thing resolves the problem, but where does it come from ?!
At last I could manage to solve this problem
I don't how i did it but now html5 video works
what I've done was verified and repare permissions several times in a row
Then I switched off and right now and I turned on again and it works!!
Launch Disk Utility. (Applications/Utilities) Select MacintoshHD in the panel on the left, select the FirstAid tab. Click: Repair Disk Permissions. When it's finished from the Menu Bar, Quit Disk Utility and restart your Mac. If you see a long list of "messages" in the permissions window, it's ok. That can be ignored. As long as you see, "Permissions Repair Complete" when it's finished... you're done. Quit Disk Utility and restart your Mac.
Carolyn Samit wrote:
Try repairing disk permissions...
It's bizarre...
Thanks for pointing out "repairing disk permissions". It fixed my problem.
Apparently, don't need to reboot after repairing permissions.