Frame-by-framing in YouTube and Vimeo!

On Vimeo:
Shift + Left Arrow
Shift + Right Arrow

On YouTube:
,
.


Sources:
https://motionarray.com/blog/learn-how-to-go-frame-by-frame-on-a-vimeo-video
https://www.quora.com/How-do-you-watch-YouTube-videos-frame-by-frame