Thank you for playing and we're glad you enjoyed it! :D
We build all of our projects in Godot 3. HTML5 exports in Godot 3 are more compatible with all types of browsers in general, especially when using the GLES2 rendering backend (which only requires WebGL 1.0). As for the performance, the textures are already relatively low resolution because of the style of our game, so there wasn't any optimisation needed.
However, as you may already know, shadows are the biggest enemy of performance. This meant before exporting the web version, we introduced an option to turn off all shadows and also lowered the default shadow atlas resolution. We also use shadow dummies for some detailed objects to give them a lower resolution shadow, which is quite funny but visually almost unnoticeable. :D
Other than that we removed any visual effects we had in the desktop version like depth of field and changed some WorldEnvironment properties like Tonemap Mode and Tonemap Exposure to match the coloring of the desktop version and the web version. If you have any other questions feel free to ask us. Have a good one! :D