This is a decent start. The basic mechanics work well.
I like that there are protected areas, especially at the start before you collect your first weapon. It would be nice it the starting point was looking at the entry to that first building to help guide where to go, rather than looking around and suddenly being attacked with no way to defend yourself.
The enemies were pretty easy to deal with, as long as you had a weapon and ammo. The turrets that shoot back fell like they should be tougher than the spiders, though with them shooting at you I appreciate that they are not harder to destroy.
After I got the second weapon (Gatling) I did not know how to change weapons. It was more useful, and I finished the level before running out of ammo, so it wasn't too bad.
I did not manage to find any weapons on the second world. I found a town where I could walk through buildings, lots of ammo, and lots of enemies. Just no weapons.
Twice the game locked up on me. It killed WebGL and refreshing the page gave me an error about not having WebGL the first time. I had to restart my browser to be able to play again. The second time I was able to replay after refreshing the page, but had also taken time to write this comment before trying, so maybe the delay helped. The first time was in world 1 as I was trying to figure things out and was pressing G to get to the menu. The second time was after I made it to world 2 and had run around a bunch. Based on the console messages I think it is memory related, but I'm not certain. I was playing fullscreen on the web when both crashes happened, so could not see the system stats. It took a while after locking up before it closed.
It is good that you have the controls on the web page, but adding a short in-game tutorial, or even signs with text giving instructions, such as the basic controls, would be nice. The second area could also use a bit more guidance on where to find your first weapon.
Overall it felt like a good base. I stopped playing due to the crashing, and would have played a bit more and tried to find the weapon otherwise.