If you don't know about HTML5, see our tutorials on Aswin's Tutorials.
Aswin's Code Editor is an app that you could write HTML 5 code and Javascript along with css.
If you want to write js, anywhere in the document add the script tag.
If you want to write css, in the head section of the document write the style tag and in between that write the css code.
Aswin's Code Editor bascially executes the given code into another page which we use the iframe tag. That code is not part of this page but on a different page.
That's How You Use Our Code Editor