I assume the plugin is just running Tcl/Tk inside a browser window.
I wonder how hard it would be to recreate the same idea using Emscripten drawing to a HTML canvas?
People have gotten Tcl to work on Emscripten [0]. I think the big problem is the Tk part. Tk has X11, Win32 and macOS backends, but emscripten supports SDL, for which there is no Tk backend.
The Projected Tk project aims to recreate Tk with a pluggable backend structure, with a web browser backend one goal.
There is an SDL backend for Tk but it's not part of the normal distribution:
I think Chrome dropped support recently too..
Would be a nice weekend hack to create a Tclet "plugin" for WASM. The Tcl interpreter itself is extremely easy to port to exotic platforms, only problem might be APIs that wrap underlying operating system APIs, those would need to be emulated on top of browser APIs.
Not sure the Tcl Plugin would still run anywhere, which is a shame