I do most of my coding prototyping on JSFiddle. It allows me to run basic CSS/HTML even JavaScript and AJAX files so I can see how the code works without worrying about images etc., and file folder management. Also it runs code really fast and gives me a look at real time design and functionality changes without rendering delays.
Here's a basic program I wrote with HTML/CSS/JavaScript that allows you to convert minutes to days. I'm going to change the design it's appalling but it shows you the basic function programs I can write.