Using the latest Safari Technology Preview Beta 5, while playing any video on Youtube, the video will, at some point, just stop playing, like the pause button was clicked - which it was not. You have to click the Play button to get it playing again. This happens with any video, and at a random time during playback. It seems to happen only once during playback of a video. Once restarted, it will play through to the end. This does not happen with the version of Safari on OS X 10.11.5
<rdar://problem/26530797>
Another point of data, it also happens on videos on other sites. For example, this one just did the same thing: https://realm.io/news/tryswift-maxim-cramer-live-design/
Steps to reproduce: 1) Open a random video like https://www.youtube.com/watch?v=cOBT_-zvUTc 2) Start watching it and try to stop the video after some time e.g. 30 seconds What happens: The video just continue to play. What you expect: The video should stop.
Sorry for last comment, shouldn't be here :) There's a workaround to continue the video if it's stops randomly and won't starts anymore: 1) Do a double right click and click on Show Controls to show the HTML5 video menu 2) Now click on the Play Button from the HTML5 video menu.
Also having this issue... Tech Preview Beta 5. Seems to happen on all html5 video on various sites.
This should be fixed on ToT WebKit so the next Safari Tech Preview should have the fix.