For the jam, I'm developing a game in HTML5, so HTML, CSS, and JS. My question is, I've noticed that when I launch my project on Firefox, there are some very slight graphical differences in some areas of my project, which aren't there on Chrome and Edge. So I have a question. If I publish it on itch.io, how can I make sure the game runs properly? Now I know that the most popular browsers are Edge, Firefox, and Chrome. I'll try opening it on Opera as well, just to be safe, but do you have any advice on this? Do you think it's a serious problem if there are slight differences? Should I mention in the description which browsers it was tested with?

The web is a tricky platform when it comes to consistency across browsers, but if it's playable, I think you'll be ok. I'm not an expert at web development, so I can't give much advice. But the main thing about the jam is to have fun doing it, so don't worry if it's not perfect. You can always fix it later if you make something you like!