I'm sorry to hear the game doesn't work properly for you. I just checked the settings of my UI Canvas, it's based on 1920x1080 and uses 'Scale with Screen Size' to keep a good UI ratio even on larger resolutions (I tested it with a 3840x2160 screen in Unity and its was fine).
Could you let me know the resolution of your monitor? That would help me figure out what the issue might be.
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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
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 ^^'