We should ship a compatibility matrix... at some point we were hoping to report test262 coverage for ECMAScript compliance, but we had to prioritize the release. Of course any of the typical web APIs work, including localStorage, and even so far as the Web Audio API (and we're working on WebRTC to make it cross platform. It's especially interesting to be able to build real-time communication apps, say, on an ESP32-S3, with just the web semantics).
In short, most app code stays inside the portable model. You need to know the target when you use host APIs that are constrained on small devices: blocking I/O, memory limits, file sizes, and of course you can combine it with native code for the target. I'm building a guitar effects processor, for example, where the UI is powered by Gea Stack but all the audio processing happens natively (https://github.com/dashersw/coyopedal).
On AI: yes, it's mostly me plus agents for the compiler. We have a team at Coyotiv that helps with everything else. I use Claude Code with Claude Opus as the main model and Sonnet subagents for parallel work, with a fair bit of Fable. I also combine this with whatever GPT model is available over Codex. During the past 6 months I've started working on the compiler, I've changed several model versions :)
What makes AI perform on a compiler:
Hard gates the agents can't argue with, like a diff gate that names every program whose output changed, plus conformance sweeps, a diary that captures past failures so they don't repeat, and treating a passing exit code as insufficient and the outputs get checked against Node.js behavior and unit tests.
The biggest differentiator for me is, even though I'm a pretty relaxed manager for humans, I'm a heavy micro-manager for AI. I read its thinking tokens and its code live and as soon as I spot something I don't like, I intervene and guide the model to the output I want to see.
It wasn't easy, Gea Stack in total I think burnt more than a million dollars in AI credits, and I've been working on it literally non-stop for 6 months.