What browser are you using? For whatever reason this game doesn't want to run on Firefox, I'd recommend using Chrome or Brave as these are the browsers I've tested the game on. I haven't tested the Mac version since I don't own any Mac PCs, only a virtual machine with no hardware acceleration
I was indeed using Firefox. Seems a lot of the games in this jam have issues with Firefox.
The Mac version not running was entirely my fault. It had been a long day and I didn't stop to think that perhaps the game needed installing first, I thought it was just a standalone. Being a Windows user all my life, I'm still not used to the interesting way OSX has you install software. Works fine on Mac!