The web build is automatically created by the the CI script.
The Web build can be tested locally by using the godot_web.py script.
Go to the directory you want to serve (build/web) and then run the script,
for example, with this command:
python3 ../../tools/godot_web.py
Then you can visit localhost:8000 in the browser to see the game.
The script is courtesy of this Reddit thread.