YouTube's CSS is apparently broken today. There are technical ways to fix it, but if you just want an easy way to fix it, put the video in theater mode. The theater mode button is along the bottom of the video screen, the second icon from the right, shaped like a wide rectangle.
This is what it looks like in theater view. It's not as nice as default view, IMHO, but it is more predictable. If you want to change back to default view, from theater view, the default button is along the bottom of the video, second to last icon on the right side, and the icon is shaped like a small wide rectangle.
