WASI Co-chair here: WASI is for the Web as well as beyond. The jco project (https://github.com/BytecodeAlliance/jco) provides an implementation of the Component Model and WASI Preview 2 for JavaScript systems. Right now, node.js support is complete, but support for Web embeddings is in progress and coming soon.
Because to me it seems only logical, to have POSIX that can work on the web.