First off, a couple of minor presentation things:
- Center the game on the screen, horizontally at the very least. Even if you're not super experienced with HTML, throwing a <center> tag on it would look a lot nicer.
- I try not to pay attention to controls given outside of the game itself because you should assume your audience isn't going to. You should find some way to convey in the game or on the page that Z does a quick drop, as it's an essential part of the game that I didn't realize existed until the spiked ceiling level.
Those issues aside, this is pretty fun. Your physics don't have any noticeable issues the immediate reset on death keeps it from being too frustrating.
Dealing with moving enemies can be pretty annoying because your own movement is so limited. The timing is simple enough to figure out, but your means of controlling your
own timing makes me feel like it's much harder than it should be.
Graphically, I have a soft spot for really simple games like this. I will say that the countdown boxes look a little out of place, as they're the one environmental object with no outline.
The levels are pretty straightforward in design and I feel like this mechanic is a little too simple for you to do much more with than you've already done. The last level in particular was a bit annoying in its design, as the bottom platform presents no challenge other than having to wait for it, and every time you die at the top you have to wait out that bottom part again. It's a small thing, but it's the only point playing this that I felt particularly annoyed by the challenge.
It's a nice game overall. Find some more mechanics to play with and start looking for interesting ways to combine them with what you've already got (the countdown blocks, for instance, seem to have a lot of untapped potential) and I think you could learn a lot about level design from making this project.