Narrow down the line of code that crashes WebGL. Go through the painfull process of pinpointing the line of code that is the source of the error. I had this "luck" that error occured when I've hit button and tried to load UIScene in Addition mode. So at first I found the script in the scene which when disabled got rid of the crash. Then I've repeated the steps and digged deeper into lines. After 2 hours of builds, comments etc I've found that the code was not working was setting the new color for UnityEngine.UI.Image. Which was pretty weird because in playmode everything worked ok.

Find solution . I think my solution might not be universal but I think there is something going in the Unity gameloop/lifecycle when running WebGL. The source of the problem was that i set the field that was storing my UI.Image in the Start method and then in some function I've tried to change the color.
Property was not exposed in inspector.

public class ModuleUI : MonoBehaviour
    Image qualityBG;
    void Start()
       qualityBG = GetComponent<Image>();
then this line below was the cause of the crash
public void Set(Module module)
         qualityBG.color = module.Quality.ToColor();
Fixing - so I've added [SerializeField] attribute to the field and set it up in editor, dragged the Image into the inspector slot. An it fixed it. I suspect that WebGL build might perform some methods in different order, or maybe the multipleScene loaded together might to do with it. I'm not sure but the field was not set properly with previous code fot WebGL build.
Bonus:
I've also fixed some issues that was not critical (not crushing WebGL game) but not working properly as expected in playmode. This also has got to do with trying to set some object in Start() method and then doing something on them. The collection was not set up (probably null) in WebGL.
EffectUI[] effects;
Start()
    effects = GetComponentsInChildren<EffectUI>();
void HideAllEffects()
        if (effects != null)
            for (int i = 0; i < effects.Length; ++i)
                effects[i].gameObject.SetActive(false);
And this also started working when I've added [SerializeField] to effects and hook it up in the scene...
Hope this will help some of you guys & gals!
To run this build within the web browser - I have removed all extra things related code those can't work within the web build like in-app purchase, advertisements, leaderboard, native sharing etc...
After these things code removed from the project, I uploaded the exported build content to my hosting. Then after it started working properly. 
                Don't you need these parts in the final product? If so, then this doesn't really fix the issue, though it is a start in narrowing it down...
– David
                Feb 20, 2022 at 19:52
        
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