Advanced overview

This section provides an overview of the platform support and other useful technical information required to build your app using WebGLA JavaScript API that renders 2D and 3D graphics in a web browser. The Unity WebGL build option allows Unity to publish content as JavaScript programs which use HTML5 technologies and the WebGL rendering API to run Unity content in a web browser. More info
See in Glossary.

Technical Overview

Unity uses the emscriptenThe toolchain that Unity uses to convert from C and C++ to WebAssembly. More info
See in Glossary compiler toolchain to cross-compile the Unity runtime code (written in C and C++) into WebAssembly (also known as Wasm). The main advantages of emscripten it’s designed to be small in size, load-time and memory efficient, and aims to deliver close to native speed execution. For more information about WebAssembly in Unity, check this blog post.

To convert the .NET game code (your C# scripts) into WebAssembly, Unity uses a technology called IL2CPPA Unity-developed scripting back-end which you can use as an alternative to Mono when building projects for some platforms. More info
See in Glossary. IL2CPP takes .NET bytecode and converts it to corresponding C++ source files, which is then compiled using emscripten to convert your scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info
See in Glossary to Wasm.

Platform support

Most popular desktop browser versions support the Unity WebGL content, but do note that different browsers offer different level of support. For example, Unity WebGL doesn’t support Mobile devices.

The following features in WebGL builds are either not available or limited due to constraints of the platform itself:

Lack of WebGL build debug support in Visual Studio

Visual studio doesn’t support debugging Unity WebGL builds. For more information, refer to Debug and troubleshoot WebGL builds.

Lack of Unity cache and caching script support

WebGL builds don’t support the Unity Cache and Caching Scripting API due to restricted access to the filesystem in browsers. Network requests to asset data and AssetBundles are instead cached in the browser cache. Refer to Cache behavior in WebGL.

Lack of managed threading support

Managed (C#) threads aren’t supported due to the lack of a multithreaded garbage collection feature in WebAssembly.

Due to this limitation, anything in the C# System.Threading namespace isn’t supported. For example, use of the System.Threading.Timer class doesn’t trigger in Web builds. As well, any timeouts specified in System.Threading.CancellationTokenSource don’t actually time out, because the cancellation mechanism is based on System.Threading.Timer.

The following code highlights these behavioral differences:

using System.Threading;
using UnityEngine;

public class NoMultithreadedTimers : MonoBehaviour
{
    private Timer t;
    private static void TimerCallbackElapsed(object obj)
    {
        Debug.Log("Timer Callback Fired!"); // This will never fire in Web builds because multithreaded timers aren't available.
    }
    private void Awake()
    {
        t = new Timer(new TimerCallback(TimerCallbackElapsed), this, 1, -1);
    }
}

public class NoCancellationTokenSourceTimeouts : MonoBehaviour
{
    private CancellationTokenSource cs;
    private void Awake()
    {
        cs = new CancellationTokenSource(0); // millisecondsDelay=0 to time out immediately
    }
    private void Update()
    {
        Debug.Log(cs.IsCancellationRequested.ToString()); // Will return false in Web builds since timeouts aren't tracked for cancellation tokens.
    }
} 

Networking limitations

There are a few networkingThe Unity system that enables multiplayer gaming across a computer network. More info
See in Glossary features that WebGL platform doesn’t support:

• Browsers don’t allow direct access to IP sockets for networking due to security concerns. For more information, refer to Web networking.

• .NET networking classes within the System.Net namespace aren’t supported.

• WebGL platform does not support native socket access because of security limitations within browsers. Therefore, WebGL also doesn’t support features like ICMP ping or UnityEngine.Ping.

Graphics limitations

There are some limitations in WebGL platform with the WebGL graphics API, which is based on the functionality of the OpenGL ES graphics library. For more information, refer to WebGL graphics.

Audio limitations

WebGL builds use a custom backend for Audio based on the WebGL Audio API, but it only supports the basic audio functionality. For more information, refer to Audio in WebGL.

Dynamic generation of code

WebGL is an AOT platform, so it doesn’t allow dynamic generation of code using System.Reflection.Emit. This is the same on all other IL2CPP platforms, iOSApple’s mobile operating system. More info
See in Glossary, and most consoles.

Copy and paste

Copy and paste works only within Unity UI(User Interface) Allows a user to interact with your application. Unity currently supports three UI systems. More info
See in Glossary. You can’t copy and paste from the system clipboard, that is, you can’t copy or paste to or from external applications.