akicode is a coding app for kids that runs and single-steps Python and C++ entirely in your browser. No server or account needed.

Underneath is a Linux clone written in TypeScript, and real Linux programs run on it unmodified, including gdb behind the Debug button.

Just for fun, you can also open the Linux terminal: xeyes and xcalc open real X11 windows, and there is even a redis-server/redis-cli, just because it works.