As the emulation is faithful to the original quirky machine, you have to be aware of some of those quirks to use it. The most important thing to know is that the machine didn't have lowercase letters, so you have to either use the shift key or the Caps Lock key to type in the letters A to Z. The emulation page has a link to a help page you can consult if you get stuck (but don't be intimidated; you don't really need to be aware of most of that stuff).
The source code is available as a zip file (759 KB), containing everything you need to run it offline. Just unzip the file and double click on ccemu.html.