A few things to check:
- Remember to make sure your canvas is set to resize based on resolution.
- For the webgl build, there's a section where you set the resolution in the player settings, check that
- When uploading to itch you have to change the game to html5, then there's again a section to specify the resolution, set it to match what you put in player settings
That's how I got my webgl not to look messed up. There is also a component you can add to your canvas to help, but I can't remember it's name and haven't worked with it, so no clue what to do there.
Best of luck to you