I have been working on a long running, but slow in development, series of posts on authoring a level creation tool using HTML5. It covers how to actually create an application using the popular MVC design pattern, implemented using the YUI3 libraries, as well as the EaselJS graphic library.
If you are interested in HTML5 application development, or in a ( simple for now ) level editor, hopefully this series is of interest to you.
Table of contents link.
Expect this to be updated over time as I continue.