This is newly possible with Pandoc 3.9, the first release to officially support WASM.

For me, too late, because my workflow is already full of little pandoc scripts that do document conversion wherever I need it!

But I wonder if the killer app might be a browser plugin, not a website. Highlight text on any webpage, convert it to whatever. Saves the copy-paste, and hopefully stays on the local machine.

Thank you for sharing.