David Ritter wrote:
Ok, after a bit of research I have found an answer for this that works for me.
It turns out that I had the "Click to Flash" plugin enabled in Safari Extensions.
If you have this plugin installed just disable it and the YouTube videos will load in the HTML5 player again.
I'm guessing the "Click to Flash" plugin needs an update to function correctly with Safari 10.
Indeed Click to Flash does not work in Safari 10. The author has stated that due to Apple changes he will not be able to fix this so effectively Click to Flash is dead and discontinued.
Note: The new default behaviour for Safari is not to enable flash on web sites. You have to explicitly enable it per website in Safari Preferences. Since most of YouTube content is available in HTML5 you do not need to enable Flash for YouTube, you would need to enable Flash for BBC iPlayer though as one example.