This is quite the question, and I'll do my best to address it.

First off, let's discuss more general Wasm programs. Currently, there are three primary modes of execution for Wasm modules - browser, node, or native runtime. The issue here is the dependence on JS - If you want to do any high-level interop between the language you're compiling to Wasm and some external library, you're most likely going to have to go through JS, pulling in a tool like emscripten to enforce the 'ABI' (so to speak) at the boundary between Wasm and JS. There are proposals, such as interface types, which may alleviate this in the future, but for now it's either go through JS or roll your own.

I'm not a huge fan of having to pull in JS for something like this, which is why I've instead been working on an alternative 'ABI' for Wasm, which supports algebraic datatypes. The basic idea is pretty simple: we hand the Wasm module a large hunk of Wasm linear memory that is encoded in accordance with the ABI. Wasm modules must expose an `update` function that takes this memory and updates it in-place. The runtime then 'deserializes' the shared memory, extracts useful data, and calls out to external libraries, modifying the shared memory (once again in-place) with any results. We toss this `update` function in a loop, run it in a background thread, and watch it to ensure it doesn't crash, write out bad data, or hang. That's the rough idea, but the actual format itself is pretty straightforward (similar to messagepack); all we need to implement is a library to serialize/deserialize this shared memory representation for each language that compiles to Wasm. All programs operate on these algebraic datatypes, so to save program state, we can just serialize the shared memory and write it directly to disk. By making a module stateless, we can easily synchronize computation and memory.

I'm currently using a hybrid operational transform / CRDT model that operates on the algebraic datatype model expressed by the ABI. Tombstones for text are largely inefficient, so we use a diff-based patch model for text inside blocks, but CRDTs for individual block-level transformations. (Wasm programs also have the ability to choose how to resolve conflicts.) Blocks are synchronized with a protocol similar to DAT, but, as stated, the protocol has built-in support for multiple writers and conflict resolution.

So, with respect to Erlang/OTP-style process migration - it's certainly a possibility. If we exposed an actor-style library to Wasm programs through the aforementioned ABI, implementing message passing would be as simple as sharing an append-only log between two computers on a network - the data synchronization would happen automatically. Spawning a new process is as simple as creating a new block in the algebraic datatype tree, and giving a Wasm module control over it. It's all pretty interesting to think about, and I'm just skimming here, but that's the basic idea.

To render out the user interface, we do something pretty simple - in addition to providing an `update` function, modules may provide multiple `translate` functions that take an ADT and translate it into another. Each node in the ADT is like a notion-style block, so all we need to do is define a `translate` function that takes the program-specific shared linear memory, and translates it to an ADT that represents a DOM (not a browser DOM, just a DOM). There are a number of optimizations that can be easily made here.

TL;DR: It's easy to synchronize Wasm state if modules are stateless, program state can be encoded as a block-based ADT, we can synchronize this state through a mixture of Operational Transforms and CRDTs, GUIs are a translation of program data.

(Woah, that's a lot of text...)