Press F12 go to the console and type `document.querySelector('video').playbackRate=0.33;`

I use the html5 video speed control extension in Chrome: https://chrome.google.com/webstore/detail/html5-video-speed-...

Mainly to play Vimeo videos at 2x speed since Vimeo refuses to put the feature in the player.

There are bookmarklets for Vimeo, and a Firefox add-on for controlling all HTML5 video speed.