I saw all, or almost all of you complaining about El Capitan 10.11.4. I have news for you.
I have a 2015 MacBook Air 11 and I experience those freezes few times per day ever since I participated in the beta El Capitan, last July. Reinstalling a fresh OS didn't help.
It happens randomly and not only in Safari. It can be any application. First, the infamous beachball of death. It will still let me switch applications, but I can't do anything, mouse clicks and keyboard press are futile. Then after few seconds it won't let me do anything at all, just watch a frozen screen. At this point, nothing but a forced power off will take it out of the freeze. If I let it alone long enough, randomly between seconds and hours, it will display the black screen of death. or not.
After restart, it will provide me a report which I've sent to Apple each time. From what I saw, each time it will complain about other application determining the freeze, and many times complaining about the video driver.
I lost hope that Apple will ever fix this, as it's happening to me for 10 months or so, consistently, every day, one to 3 times a day..
later edit: one more thing worth mentioning is that it doesn't even need human interaction to crash. sometimes I let it in running/sleeping in the office and in the morning I find it frozen in the black screen of death, and very warm, if not hot.