Back in 2014, I found a way to loop YouTube videos using a feature of the HTML5 player. It looks like YouTube added "loop" as a standard feature. Just right click inside the HTML5 player and click "loop". This way, you can watch a video on repeat until you pause it or uncheck "loop" from the contextual menu.
It also works for embedded videos. Here's a short video you can use to test the loop feature: