How to stop HTML5 autoplay in Safari

if can tell me how prevent safari autoplaying html5 videos webpage opened, i'd grateful.
irritating indeed.
@ least clicktoflash, user can decide flash videos play , when. surely there must similar feature html5?

 

if youtube have problems with, unsubscribe html5 beta @ www.youtube.com/html5 .

maybe have extension installed, forces html5?

 

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