The emulator is a native-client application. It only runs on the Chrome browser and you must enable native-client apps by visiting the chrome://flags/ URL, enabling native-client apps and then quit and restart your browser.
You can drag d64, t64 or prg files for the Pet, Vic20 or C64, also cartridge files (.20, .60 or .a0) for the Vic20 and dsk files for the Apple][ directly onto the emulator. Use the function keys for the top row of PET keys !"#$ %'&\ ().
You can also resize the window by clicking and dragging the indicator near the bottom right corner of the emulator window (only works with hardware accelerated rendering).
You can break into the debugging monitor by pressing SHIFT and PAUSE. Press F10 to step over, F11 step in and F5 to run. Monitor display commands are based on the monitor in the Commodore PET.