What, what? YouTube has been doing an HTML5 based web player for quite some time now[1], and I believe it supports FLV video as well, though I can't find a reference for that.
I have uninstalled Flash from my system. A great many videos on YouTube give me the "you need the latest flash player to watch this video" error.
Yes, several years ago, I was getting MP4 video, and today, a portion (say %30-%40) just launch with MP4. But most of the time I get the "you need flash".
It's really weird. Using the built-in HTML5 player, half the videos don't load. But those videos do exist — Google just doesn't want you to be able to access them easily. I can watch them on my iOS devices, I can download them using dirpy.com, and I can watch them in my browser using the YouTube5 plugin.
[1] https://www.youtube.com/html5?gl=CA