Youtube on Safari won't zoom enough

I'm on youtube on Safari on my Mac. I'm on "View Actual Size", and the browser is taking up my entire screen, and yet, for some bizarre reason, the width of the content is exceeding the width of my screen, so that I constantly have to scroll back and forth to see all the content.

Safari allows me shrink the content through "Zoom out", and this reduces the problem somewhat. But after 3 clicks on "Zoom out", the feature becomes disabled, and I cannot Zoom out anymore, and the width of the content still exceeds the width of the screen?! Every other browser first of all doesn't have this problem to begin with, and secondly, let's me zoom out as much as I want!

And yet another problem: related videos shows only a thumbnail -- no text! Again, only on Safari?!

Going into theater mode solves the problem -- but I don't like theater mode -- I now have to scroll down every time I want to see related videos, a major inconvenience.

I've been using youtube for about a decade, and this is the first time I've ever had any such silly issues with it, all thanks to Safari. :(

Safari Version 8.0 (10600.1.25) On a Mac OS X Yosemite Version 10.10 iMac 27-inch