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Select a youtube resolution and resize the player.
You have to modify the targetRes in script manager's storage, not the script itself.
Hi jonatino, sorry for a late response. I'm not having any issues with quality changes being applied myself. Like partment said, you either have to change the setting in your user script manager or set overwriteStoredSettings to true for your changes to have effect. This was done to avoid overwriting people's settings during updates. If there is some other issue with it, please provide which browser and userscript manager you are using (and an example URL could be helpful too) so I can try to reproduce.
No longer working on youtube. Doesn't change quality at all anymore.