Youtube proper does a lot of JS, eg. a view is usually only counted after ~30 seconds[0] and does other things like logging the current timecode so that it can return to that time in case the user clicks off the video and returns to it later[1].

0: https://www.tubics.com/blog/what-counts-as-a-view-on-youtube....

1: https://i.judge.sh/rare/Cherry/chrome_H0p9bAoLgy.png the red line is how far in i've watched.