Re:Chrome will begin pausing many Flash ads by default on Se...
by TazeTSchnitzel on Friday August 28, 2015 @03:05PM
(#10137314)
Mobile Safari did this from very early on! (iPhone OS 2 or earlier) Though they didn't use YouTube's official HTML5 player.
https://github.com/hfiguiere/no-flash/
Mozilla may include this functionality in Firefox directly:
https://bugzilla.mozilla.org/show_bug.cgi?id=769117