At first I was having trouble with the menus of the game, not understanding well how to save and deck build but then I realized it was actually a resolution problem. My screen wasnt set to correct resolution so the menu weren't working probably. I used zoom in and zoom out in my browser to get the right resolution going.
This is a nice concept for a game, I had fun playing it (and I believe I played it fully? There wasn't an end screen or anything I just beat the third boss and couldnt anywhere else to go). I would love to see more from it, with art and music done for it, would be a good idea to see fully flashed out.
The deck balance is a little uninspired, because I had little reason to build anything other than what was working for me, maybe thats just how my playstyle was, or the balancing was just a little off. Would be cool to see some cards interacting with each other more, like if you get scatter shot instead of changing your gun to a shotgun type of weapon, make your assault mix with scatter shot or something like it.
I really enjoyed the metroidvania aspect of it, although it was hard to orientate sometimes, mainly because of the simplistic artstyle I feel like!
I had to reload the save file because I didnt know the mechanic of using opponents missiles to destroy armor, so it would be nice to have a more intuitive tutorial, like force the player to learn this mechanic before advancing to an area where you need, somehow.
Ok apparently there's a bug where the first boss door refuses to open for some reason, reloading seems to fix it.
Also, it would be nice to have separate menus for the save and for pausing the game. Trying to quit the game while you stand on top of a save point is not an option and its a little frustrating.
Good luck on your game dev journey, friend! :)