Set aspect ratio in fullscreen WebGL?

If someone needs a solution for this I got it to work this way:

Add a second Camera to the scene with a solid color background set to black and its depth lower than your main camera.

Add an Aspect Ratio Fitter component to any of your canvases and set it to "Height Controls Width" and 16/9.

And add the following script I found online to your mainCamera to keep its aspect ratio always at 16/9.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class KeepAspectRatio : MonoBehaviour
 {
     void Update()
     {
         // set the desired aspect ratio (the values in this example are
         // hard-coded for 16:9, but you could make them into public
         // variables instead so you can set them at design time)
         float targetaspect = 16.0f / 9.0f;
 
         // determine the game window's current aspect ratio
         float windowaspect = (float)Screen.width / (float)Screen.height;
 
         // current viewport height should be scaled by this amount
         float scaleheight = windowaspect / targetaspect;
 
         // obtain camera component so we can modify its viewport
         Camera camera = GetComponent<Camera>();
 
         // if scaled height is less than current height, add letterbox
         if (scaleheight < 1.0f)
         {
             Rect rect = camera.rect;
 
             rect.width = 1.0f;
             rect.height = scaleheight;
             rect.x = 0;
             rect.y = (1.0f - scaleheight) / 2.0f;
 
             camera.rect = rect;
         }

         else // add pillarbox
         {
             float scalewidth = 1.0f / scaleheight;
 
             Rect rect = camera.rect;
 
             rect.width = scalewidth;
             rect.height = 1.0f;
             rect.x = (1.0f - scalewidth) / 2.0f;
             rect.y = 0;
 
             camera.rect = rect;
         }
     }
 }

if you are not looking to change the aspect ratio once the game is running you could obviously put that code into void Start()

I think adding an Aspect Ratio Fitter to you Canvas is causing your Scene to be marked as dirty. Instead of add Aspect Ratio Fitter to your Canvas, create a new empty Game object below the canvas group and move the Aspect Ratio Fitter to the new empty Game object.

see https://support.unity.com/hc/en-us/articles/215620303-Using-an-Aspect-Ratio-Fitter-causes-my-Scene-to-be-marked-as-dirty