Well, there's a lot to unpick here.
It's an okay *base* for a game. There's a player who can shoot, a way to replenish ammunition and health, there are enemies and destroyable barriers… genuinely a lot, but not without some serious flaws.
The largest problem of the game, as I see it, is rotating the player and the enemies. The center of the rotation isn't in the center of mass, but rather at the tip of the gun/arms – which means whenever you rotate, you change position, a lot.
For the player, this results in unpredictable movement and for the enemies, this results in them getting stuck in walls. As a rule of thumb, characters who are able to turn on the spot shouldn't change their position when doing so.
The second problem is the lack of sound effects or music. There's not much to add, it simply throws a good portion of the game's atmosphere out of the window which lessens the overall quality and makes its flaws more visible.
Also when many enemies get stuck inside each other, they all harm you on touch. In other words, when you have three hearts left and run into three inter-locked enemies, you die.
It's a pity there's no way to restart the game if you die, you have to reload the page.
Since ammunition is scarce, you have to be careful not to waste it. It's not a problem that zombies kill you when you're out of ammo, but you can also be blocked by a box – at which point you didn't die, but also there's no way to continue.
The bullet exploding animation seems to have a frame at a different position.
And unless I'm mistaken, there's no victory screen either which makes the game seem unfinished.
It's a good start, but I think it needs a bit more work until it could be an enjoyable experience to play.