Cannonball is a project akin to Webkit for ActionScript 3.0. It is an implementation of the DOM Level 3, HTML and CSS3 core specifications for ActionScript 3.0.

The CSS package is the most complete and provides:

• support for many of the CSS3 selectors, based on The Simple API for CSS
• an API for exposing any object as a CSS-stylable node
• style cascading with respect to document origin, selector specificity, and declaration priority (!imporant)

You can explore the CSS selector engine here.

For more information on CSS3 selectors, check out the W3's Selectors Level 3 working draft. It was updated March 2009 and is a last-call working draft for the specification.

You're welcome to contact me at john at newgonzo dot com.