I had a similar issue with Unity6, where it would not resize at all. I found this really cool publishing profile which is simple to drop into the project. If you leave the default it will maintain the 16:9 aspect ratio you have made the UI for. But my resolution is not normal (7680x2160), so yours might be fine on a standard ultra wide. https://seansleblanc.itch.io/better-minimal-webgl-template
Viewing post in MoonSweetHome jam comments
I tried to make a new build using the 'Better Minimal WebGL Template', but it added a strange loading bar and black horizontal bars on 16:9 screens. So I reuploaded the game (with the Better Minimal Template), but without changing my submission.
If you want to try it: https://chatnoir95.itch.io/mshbetterminimaltemplate
I hope it will work ^^'