The four designs of the Pentagon language competition: RED and GREEN, the finalists of 1979, and BLUE and YELLOW, dropped after the first phase in 1978. Your program is translated to C by a translator written in its own language, compiled by clang and run here, with a thread for every task. Nothing leaves the page.
redc, greenc, bluec and yellowc, the translators written in RED, GREEN, BLUE and YELLOW, are built to WebAssembly from the same C seeds the repository bootstraps from. They translate your source to C; a YELLOW program gets the standard prelude in front of it, as on Linux. clang and lld, compiled to WebAssembly, turn that C into a program for wasm32 with threads. The runtime is the same C file as on Linux: it hands the processor from task to task by the RED rules, on a virtual clock, so the output here matches a native run byte for byte.
The first run downloads the C compiler, about 20 MB, from jsDelivr; after that the browser keeps it.