WASI
WebAssembly System Interface — a standardised API that lets Wasm modules talk to "the system" (files, sockets, clock) in a sandboxed, portable way.
Originally from Mozilla (2019). Built on capability-based security: modules don't get resource access implicitly, the host runtime must explicitly hand in a file handle or socket. WASI Preview 1 (snapshot1) is what most runtimes support today; Preview 2 (2024) builds on the Component Model with richer interfaces (HTTP, key-value, sockets) and is where runtimes like Wasmtime and Wasmer are headed.
Makes it viable to run Wasm outside the browser — in edge functions (Fastly, Cloudflare), plugin systems (Envoy, Shopify Functions), and as a lightweight container alternative.