YouTube was built around Adobe's Flash Player. Over the past few years, YouTube (and parent company Google) has been moving away from Flash.
Besides being filled to the brim with security holes, Flash Player is an inconvenient add-on to most Web browsers. YouTube will now stream videos through HTML5.
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For those who aren't familiar with programming, HTML is the building block for almost every website. HTML5 added media and interactive features to the programming language.
Besides being more secure than Flash Player, HTML5 also allows YouTube to improve video quality and streaming speed for its content.