When you "Build and Run" a Unity WebGL build, Unity internally serves your build with a static local web server. Usually the address is something like "http://localhost:29531".

It sometimes can be painful to relaunch this server. The workflow that I've seen has been to just "Build and Run" again from Unity, but this can take a long time. This can be especially painful if you're trying to modify the WebGL template.

A better workflow

Instead of relying on Unity's local server, you can run your own. Sounds scary, but it isn't really. There are a few ways to do this, and requires a little bit of command-line, but no coding. First you can determine if you already have the required software (either python or node.js).

Node.js is my preferred method, but since python comes pre-installed on a variety of systems, it might be worth trying first.

Python

From the command line try:

python --version

If your version is 2.x.x, simply cd to your Unity WebGL build directory and type:

python -m SimpleHTTPServer

If your version is 3.x.x:

python -m http.server

Either of these commands will usually print something like:

Serving HTTP on 0.0.0.0 port 8000 ...

That means you can go to localhost:8000 from your web browser to test out the build.

If you get a command not found, you can either install python or try the node.js version below.

Node

To check if you have node installed try the following from the command line:

node --version

and / or

npm --version (npm is the node package manager that comes installed with node.js).

For reference, I have node v10.16.0 and npm 6.9.0, but the version shouldn't make a huge difference unless you're using a really old node version like <4.0.

If you don't have node, you can install it at nodejs.org.

Once you have node and npm, you can install the "serve" package globally:

npm install -g serve

Finally, cd into your webgl build directory and type:

serve

which should output:

   ┌─────────────────────────────────────────────────────┐
   │                                                     │
   │   Serving!                                          │
   │                                                     │
   │   - Local:            http://localhost:65333        │
   │   - On Your Network:  http://192.168.86.239:65333   │
   │                                                     │
   │   This port was picked because 5000 is in use.      │
   │                                                     │
   │   Copied local address to clipboard!                │
   │                                                     │
   └─────────────────────────────────────────────────────┘

In my case, I'm already running a local server on the default port (5000), so here, I would go to http://localhost:65333 in my browser to display my Unity WebGL build.

Shutting down

Usually ctrl+c will shut down your server, or you can close your command line window.