The way this works is by creating a secure context through a service worker running in the background, which allows Godot to use SharedArrayBuffer to communicate between Threads etc.
Which means you don't have to have the headers if you can't configure them for your hoster (although if you can that's still preferable as there is probably some JS overhead at least when loading the page with this).
I have managed to deploy my game to Gitlab Pages through the use of the godot-coi-serviceworker addon from the Godot addons library. Should probably work as well for GitHub.
Just keep in mind that GDScript is actually a different programming language, so things like method names, libraries etc. don't carry over. It mainly shares the syntax of having no {} braces and semicolons, as well as using indentation to group statements after a colon (:).
So while it's definitely helpful to learn programming fundamentals like if statements, for loops etc. in Python, you could do the same in GDscript with a similar experience (but maybe a little less learning materials available).
I can recommend GDQuest's interactive GDscript tutorial that takes you through all of the basics and is more hands on than just reading/ watching lots of tutorials:
Generally it's fine unless you use very complex meshes or a huge amount of simple ones. One thing to note is that CSG is mostly meant for quick level prototyping, but once you are done with that you can convert the result to an ArrayMesh to save on calculations and improve the performance.
You can then even export the resulting mesh to glTF/ glb and edit it in Blender etc. for a polish pash, which is a pretty nice workflow since it gives you both the quick iteration time of building your levels directly in the engine as well as the extensive set of tools that comes with Blender or other modelling tools.
The way this works is by creating a secure context through a service worker running in the background, which allows Godot to use SharedArrayBuffer to communicate between Threads etc.
Which means you don't have to have the headers if you can't configure them for your hoster (although if you can that's still preferable as there is probably some JS overhead at least when loading the page with this).