How to build with Emscripten
Building with Emscripten works with emsdk 3.0.0 and above.
You will also need libharfbuzz-dev and libicu-dev installed on your host OS.
sudo apt-get install autoconf automake libtool gperf libharfbuzz-dev libicu-dev libfreetype-dev
./emscripten-build.sh
And now you have in your build folder files like "openttd.html".
To run it locally, you would have to start a local webserver, like:
cd build
python3 -m http.server
Now you can play the game via http://127.0.0.1:8000/openttd.html .