Google started it's quest with HTML5 player for YouTube in 2011 when Adobe discontinued flash support for mobile devices. But with limited browser support and relative immaturity of the HTML5 player, it was no match to the flash player at that time.

With the advancements over time, both are now at level grounds at least in all major browsers. For YouTube, the HTML5 player was kept as an opt-in for all browsers. But now, Google is enforcing HTML5 to be the default player without choice on Chrome. That means if you try to watch any YouTube video in a Chrome browser, it will be played using the HTML5 player always.

Disable HTML5 Player and use Flash in Chrome

If you have a strong reason not to use the HTML5 player, here's how to do it -

1. Chrome.
You can force Chrome to use the flash player by installing the "Disable Youtube HTML5 Player" extension from chrome store. After installing, you can right click on any playing YouTube video to check that flash player is indeed getting used.

How it Works: YouTube relies on a cookie to switch to HTML5 as part of their original opt-in feature. For chrome alone, this cookie is always set and the opt-in page doesn't give an option to un-set it. The extension explicitly unsets it fooling YouTube into delivering using the flash player!

2. Firefox, IE and others.
In all browsers other than Chrome, YouTube uses the default player from the browser settnigs. But if you find that YouTube is not honoring that, please check if you have accidentally opted-in for the HTML5 trial and opt-out.

HTML5 vs FLASH player - which one to use?

For the most part if you don't have any issues with the way YouTube videos play currently, just use what ever YouTube is defaulting to. HTML5 player has the added feature of speed control and slightly lesser CPU usage. But everything else is pretty much the same in both players.