The concept is not bad. But I find it is a bit hard to tell when will I be jumping. When I thought I would jump but I didn't. Also, I felt that the jumping strength is not so accurate as sometimes I couldn't jump higher even if I hold longer or even don't jump at all.
Also, I think it would be good to have a restart button or back to starting page button at the end of the game to restart the game instead of manually refreshing the whole web page.
But I think overall the game is not bad. Good job!