Using "Z" as a key makes it really hard for Germans to play the game since for us, y and z are switched on the keyboard.
Using x,c,v or 1,2,3 would be better in this case :)
Other than this: fun game
Other than this: fun game
1,2,3 + arrow keys would be a better choice for sure.
The reason that this isn't a problem for Flash is because Flash grabs and steals all keyboard and mouse focus away from the browser; not something anyone should re-implement in JavaScript.
Seriously, if you haven't done it already, find or make yourself a module that lets the user customize the keyboard controls to his/her preference. Then you can just drop it into any game you write, and presto! You're good to go!
For even more gooey goodness, make up some one-click "profiles" for different keyboards that users can pick at a click. You don't need to make it top priority, and you may not even need to do all the work yourself. If you just ask for people's preferences in a forum, or a feedback form with the game itself, you can probably get a ton of helpful input.
Best of luck!