Hi, It appears a number of people have this issue and, as it's frequent, you'll likely know exactly what I'm talking about or not have a clue if you're a lucky one that doesn't. Basically, when watching HTML5 videos in Safari or Chrome (I haven't tested Firefox), the system will randomly freeze. For me this is triggered 90% of the time by closing or switching from a YouTube video to another. If it freezes when I close the tab it will quite often do so during the animation of enlarging the other tabs to occupy the space left by the closing tab. In this case, if I wait about 2 minutes, it will unfreeze. However, if I switch tabs and it freezes, it rarely unfreezes. Most of the system is unresponsive. The time in upper right corner remains stuck, no keyboard commands or mouse clicks work. I can't force quit anything, open menus, swipe, or do anything other than a hard reset. Interestingly, my mouse cursor will still move, whether from the trackpad, USB of Bluetooth mouse. Music that was playing will continue to play. So it feels like it's the UI that's frozen and most background tasks will continue. I can't access any background tasks so it still results in a hard reset. 10.11.2 has not fixed this problem. It's also introduced another. I can no longer maximise YouTube videos. The screen darkens, the controls move to the bottom but neither the picture or the controls expand to fill the screen. I can shrink them again and full-screen them again with the same issue. It has also caused my system to freeze after switching to another tab. As I've read about lots of variations of this HTML5 video freeze and not seen it addressed by Apple in remedies or in the 10.11.2 bug fix, I'm now worried it's a hardware issue affecting a number of their devices. I've a 15" rMBP from mid-2014. Support just told me to try resetting the NVRAM, SMC, etc. and wipe the system. The usual suggestions, but didn't mention any familiarity of the problem or any direct solutions, so I'm unsure if they're being disingenuous as there's no outright solution yet. If you Google it it's certainly happening quite a few people.