There's a very bad bug in this.
If you start from the very beginning and play the levels sequentially, you won't be able to play the last few levels at all and you'll need to either clear browser cache and play again, this time starting from the end going backwards *or* start again in another browser, again from the end going backwards (just to be sure)
What happens is, if you play it sequentially, the later levels start disappearing from the level select altogether as you start clearing stages.
That is an extremely frustrating bug!