[proveanegative]

>browsers could recognize YouTube's boilerplate embedding HTML and rewrite it with YouTube's HTML5 video.

Browsers themselves don't need to do it. It could be a (very useful) browser extension or a user script.

[threedaymonk]

Here's the user script I use (I've not allowed Flash on my computers since about 2010):

https://github.com/threedaymonk/userscripts/blob/master/yout...