I have been experiencing some glitches when watching full screen videos from YouTube, Vimeo and similar sites, but only when they are embedded on others websites. It does not happen when I watch the video inside Youtube, Vimeo, even on full screen. It only happens on Safari, Chrome works perfectly well. I am not sure if it is related to El Capitan or the MacBook I am currently using (MacBook Pro Retina 13, i5, late 2013 with Intel Iris), I asked some friends to open the same URL and do the same test, they could not reproduce those glitches. My main concern is about the source of the problem, if it is just a Safari bug or it may be related to my graphics card. Tried to reset the PRAM and SMC and it did not fix the problem. Here is a video where I show exactly what's happening. Hope anyone here can help me.