Example: https://greggman.github.io/better-unity-webgl-template/
Download and add in this unity package
into your project. Then pick Edit->Project Settings->Player, in the WebGL tab
- Change Player/Resolution and Presentation to "Better2020"
- Change Player/Splash Image/Splash Style to "Dark on Light"
- Change Player/Publishing/Compression Format to "Disable"
You can also set "Hide Full Screen Button" to 1 or true if you don't want the fullscreen button to appear
in the bottom right.