WebRTC data channels would be ideal, so that clients can connect directly to servers (or at least regional proxies) in a UDP-like way. Vanilla minetest would need to include an WebRTC endpoint for this to become universally available. There are lots of details that would need to be worked out.
A handful of key combinations cannot be cancelled. With the Full Screen API, the page also loses use of the ESC key, which is extremely important to Minetest's menu interface. (That's why F11 is the recommended way to go full screen)
For storage, a bare minimum Minetest world needs 1GB, grow-able to multiple gigabytes. I don't think IndexedDB is meant for that.
The network issues are specific to Firefox. Multiple threads in Minetest do network I/O. In Chrome this works fine, but on Firefox only the main thread can do I/O, and so network requests are proxied to the main thread (this is done transparently by Firefox, using a mutex internal to firefox). Unfortunately this arrangement leads to a deadlock given the particular way it is used.
There is definitely a tremendous amount of work going into emscripten/wasm. Minetest is multi-threaded, and so relies on SharedArrayBuffer which only became available in 2020.
The UX issues do seem to be stubbornly persistent though. I ran into the same issues long ago porting games with NaCl and asm.js. I'm not sure what is preventing progress there.
Hi all, I made this. Surprised by the sudden interest. It has been live but unchanged for 6 months.
There's an updated launcher. It's not finished yet, but it is nicer for exploring single-player community-made games: https://minetest.dustlabs.io/ez/
I'm not actively working on this, as I have concluded it is not (yet) possible to create a good web experience. There are problems with storage, network, keyboard side effects (e.g. Ctrl, W), concurrency, and performance that need to be addressed by browser vendors. I highly recommend installing the native Minetest!