Tells YouTube that your browser only supports videos at 30FPS or less, which switches all 60FPS videos to 30FPS and allows old computers to watch high-resolution videos without stutter!
< Disable YouTube 60 FPS (Force 30 FPS) 피드백
Also to add something. Even if you use a program like jDownloader 2 to download the video, which allows you to choose which resolution/FPS/Audio quality video you want, there will be no 1080 FPS for the videos I mentioned above. Just to help some people understand that there is no way anymore to get the 1080p 30fps video.
Just to clear out some confusion, the script still works as intended. It is not that the script does not display 1080p 30FPS, it is that YouTube removed the 1080p 30fps videos, at least if the uploader used a 60 FPS video.
For example, if you check "YouTube 60fps Tester" (/watch?v=Cyxixzi2dgQ), which the original uploaded video was 60FPS it will only have 720p with the script. If you though go and check "30fps judder test" (/watch?v=-Ht8EIAOrj8) that the original uploaded video was 30FPS, it will still have 1080p.