Hello
It looks like you're using the wrong Quality/RP Asset when building for WebGL. The one that is chosen for WebGL is the Mobile_RPAsset, which uses the Mobile_Renderer asset. That Renderer does not have the Decal Renderer Feature enabled.
You will have to add the Decal Renderer Feature to the Mobile Renderer, or change the WebGL build to use the PC_RPAsset under Project Settings -> Quality.