You shouldn't. Twitch is still holding onto Flash. They could have really leveraged their position as the most used streaming site to push for adoption of HTML5 streaming. They have a HLS mode, but it's only supported on Safari/Mobile/Edge.
[1] http://blog.twitch.tv/2015/07/video-player-controls-now-in-h... [2] http://www.engadget.com/2015/09/25/twitch-html5-whisper-twit...
Of course Twitch should support HTML5 video though, no idea why they aren't.
They are, in the meantime, settings things up to make the transition easier, e.g. moving the video controls out of the video, moving chat out of flash, etc etc