I'm getting the same problem with Safari and Firefox but not on the Apple site. One page in question has a HTML5 animation that plays a few seconds of audio on load. The audio plays, so I know the content is there.
I've got a corporate website with quite a few pages and a number of HTML5 animations. Some pages that are showing up as blank have been up and running without any changes for most of a year but are suddenly not working, but only in Yosemite. Expecting consumers to lift a finger to view a page is a non-starter.
I've wiped the cookies, cleared the caches, relaunched, etc., but the problem remains. I assume that this is not a browser problem but that it is because of a change in rendering that was introduced in Yosemite.
It would be nice to hear something from Apple on this one.