Youtube now defaults to HTML5 video player for all browsers:
Here is one more nail in Flash’s coffin: starting today, YouTube defaults to using HTML5 video on all modern browsers, including Chrome, IE 11, Safari 8 and the beta versions of Firefox.
via YouTube Now Streams HTML5 Video By Default | TechCrunch.
Youtube began supporting 3D videos and output devices back in about 2009 using a Flash-based video player. HTML5 does not presently support 3D video output options. Youtube has made no official announcement as to what will happen with 3D support.