There is a bit of math to calculate where the mouse is moved over a bar that will control the video speed. It is a good idea to practice lots of things that use this recording of where the mouse is or where something is clicked. The math is not rocket science, but understanding the tools that are needed to get positions are important. I have read the docs for MouseEvent.pageY, Element.offsetTop, and Element.offsetHeight quite a number of times now and while they are also not complicated, I feel like I can do with even more practice with this. I'll cook up some codepen at some point where I illustrate what they do so I can use that as reference.