a user script that can be used with greasemonkey (firefox) or tampermonkey (chrome) to add keyboard shortcuts that will increase/decrease playback rate of video elements.
first, grab the user script from either
http://userscripts.org/scripts/show/160029
or
https://github.com/mirnhoj/video-element-playbackrate-setter/
then add the user script to greasemonkey or tampermonkey. after that, you should be able to navigate to a web page with a video element and adjust the playback rate. use the open bracket key [ to decrease the playback rate or the close bracket key ] to increase the playback rate.
optionally, you can alter the source code in greasemonkey or tampermonkey to apply the script to other places besides youtube. you can also easily change the default playback rate and rate adjustment granularity.
don't forget about the standard youtube player's keyboard shortcuts. these are the ones that seem to be working:
| keyboard shortcut | action |
|---|---|
| space bar | toggles play/pause |
| , . | skip backwards or forwards a frame |
| ← → | skip backwards or forwards 5 seconds |
| ↑ ↓ | increase or decrease volume by 5% |
| home end | skip to the beginning or end of the video |
| 0--9 | skip to 0%--90% of the video |
you can also see a list of keyboard shortcuts on one of youtube's help page (http://support.google.com/youtube/bin/answer.py?hl=en&answer=189278), but these seem to be not entirely accurate.