Took me a few minutes to get the sequence of setting up a loop but it's working nicely right now on a YT vid I'm trying to learn a piece of music from...
R click in YT window and select Loop
With vid paused got to your Start point
R click twice to bring up the menu with Custom Loop options
Select "Select Start"
Move video to End point
Select "Select End"
Hit Play and the Vid will loop.
If you are past the start point the vid will jump immediately to your Start Point
If you are before the start point the vid will play from there, up to the End point Then start looping.

Very good, very useful. Right I'm back off to learn these notes ;^)