The C source of GONG RIDER is on the left. Change it and press Build & run: the page preprocesses, compiles, assembles and links it into a Vectrex cartridge in about 80 ms, with CMOC and lwtools running right in your browser, and the Vectrex on the right plays your version. Only want to play? Press Play.
Each one changes the code for you. Then press Build & run.