The independent Google Operating System blog took note of the improvements to YouTube’s HTML5 player. The HTML5 interface is in an opt-in trial period that you can enter and leave at will. Once you opt-in, Adobe Flash becomes unnecessary when using a browser that supports HTML5. The question I had and answer here is: How does the YouTube HTML5 interface work with mobile browsers?
The Google Operating System blog noted that YouTube HTML5 player improvements include the ability to enable annotations and captions, a contextual menu to let you copy embedding code and a video’s URL, 480p and 1080p options for WebM videos and full-screen support when using developer builds of the Firefox or Chrome browser.
The YouTube HTML5 player provides a mobile friendly interface too. This worked on the iPhone, Android and Windows Phone 7.5 (Mango) devices I tried with it. The mobile interface allowed giving a video a thumbs up or down (if enabled), adding to a list, sharing via Twitter, Facebook or email and deselecting high quality (HQ) mode (which is the default when available). Most importantly, full screen video viewing worked in the mobile browsers (including the Windows Phone 7.5 Internet Explorer browser). The controls you see in the second screenshot here appear when you tap the display while a video is playing.