From the Godot documentation (https://docs.godotengine.org/en/stable/tutorials/rendering/renderers.html): "In this case, Compatibility is the only choice."
So, yeah, apparently you'll want to use Compatibility, not Mobile or Forward+. Neither of the others are meant to handle HTML5.
Also, if you want to make sure your game will run properly on the web, it's a good idea to go into the Export settings (from Project > Export...) and add a Web export preset to the menu prematurely (you might have to wait for some export templates to download first). After doing that, a new "Remote Debug" option will appear near the Run Project button, and you can use this to run a debug version of the game in your browser, just to test that the HTML5 will work before doing the actual export.
I would highly recommend consulting the documentation for anything else you need help with, since it explains much of the process way better than I can.