I am unable to watch youtube videos on my IMac. the o/s is OS X El Kapitan V 10.11.5.
This problem started about 6 months ago and I found I could get around it by selecting several other videos and finally one would work. Don't know why. then I would back up to the one I wanted to watch. The number of videos I had to select increased over time.
Now I cannot get anything I want. I get audio, can see the individual pix on the "time line", and the center of the screen will sometimes have an F and another character superimposed. And circles rotating around it which I had interpreted as download in progress.
I tried a MAC site and clearing history was the only suggestion I got.