It blows me away that YouTube hasn't implemented this feature themselves, so I've go to thank the author of Yays! for making this functionality available. I picked it up for the ability to disable AutoPlay, but the fact that it also lets you auto-set the quality of all videos to max is amazing!
My only gripe is that it doesn't work when my other favorite script, YouTube Enhancer, is installed. But autoplay and auto-max quality are better features than what I use TYE for, so I don't mind disabling it until I need one of it's features.
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