(Errors from your JS or Pre-Processors code)
In the HTML box, you only need to write the code that belongs in the <body> tag.
If you want to put something that must go inside the <head> tag then you can simply write full HTML that starts either with a doctype or with a <html> tag.
You can use tools like Prefixfree, Normalize, and other Preprossers like Jade, HAML, Stylus, Less, SASS, Coffeescript and much more!
Take a look at the Preferences Pane on the left to see all available tools.
Zen Coding in HTML and CSS is also supported!
Code font-size can be adjusted! Code box alignment can also be changed to horizontal or vertical positions.
Need more JS resources? Try cdnjs.com