After the release of HTML5 and CSS3 standard for the web, the face of many websites is changing as it supports embedding videos with <video> tag just as how we are using the <img> tag. 

YouTube and HTML5
YouTube and HTML5
The majority of websites masters is stopped using Adobe flash player and using HTML5 player instead.

YouTube, the biggest place where Adobe flash player is residing, is thinking about changing to HTML5 video player instead of Adobe.  
But why still YouTube is not using HTML5 video player as primary player is, lack of support for earlier versions of Web browsers (your browser should be updated for using HTML5 player).

Even though YouTube has released HTML5 video player version and users have to check compatibility and enable it manually.

If you are not satisfied with HTML5 player, you could move back to the old YouTube player at any moment. 

How to Enable HTML5 video player on YouTube

You could check the compatibility of your browser for  YouTube's HTML5 version here. The only conditions to be qualified for using latest HTML5 version is, your web browser should be updated. If HTML5 is supported by your browser, consider updating it.

HTML5 compatibility check for YouTube
HTML5 compatibility check for YouTube

If  your browser supports HTML5 player, go to https://www.youtube.com/testtube and enable HTML5 player by clicking the button "Get Player".

Enable HTML5 video player for YouTube
Enable HTML5 video player for YouTube

If you are not satisfied with HTML5 player, you could move to the old player by visiting this (https://www.youtube.com/testtube) page again.

YouTube with HTML5
YouTube with HTML5