I've set my viewport dimensions, I've messed around with scaling and viewport size in Godot, and yet it still shows up wrong.   The first image is how it shows up on itch.io, the second image is what it's supposed to look like.

(This is in Firefox, so "don't set it to open in a new window" is not an option.  Edit: Tried it in Chrome, where I can set it to run on the main page rather than opening a new window, and it does the same thing)

Any ideas of how to fix this?

Hello! I had the same problem and managed to solve it. I'm not sure if this is going to work for you, but it's worth a try:

When you're going to export for web, go into the Options Tab, look for "Canvas Resize Policy" and make it "Adaptative". Hopefully, this may solve your problem.

That did it!  Thank you!