Do I understand correctly, you want to make a (X)HTML+CSS renderer in Flash. Right ? 
What kind of (X)HTML do you aim for ? Excuse my ignorance but, is Flash capable of generating a valid XML tree from any crap found on the net ? Otherwise you'll have to limit yourself to valid XHTML or code a loose parser and that's not funny.
Even if you limit your renderer to margin, padding, font and color, there's quite a few ways these properties can be set : inline style, style in the head, imported style, cascading, a combination of all of that. And yet parsing these properties is just the tip of the iceberg. How do you render them ? the standard way ( with margin-collapsing, normal box model, ... ) or in quirk mode.
You mention Print, do you want to parse stylesheets for the Print medium or apply the default one ?
Well I'm sure it's technically possible regardless of the language but for a second, try to imagine the amount of man years put in browsers like IE, FF, OP, SF. Even if say 5 persons worked on the parser and layout engine, that represent more than 50 man years of work.
I don't have any Flash ressource, but the (X)HTML and CSS standards are open and the specs are all available on http://w3.org
May I ask why you want to do this ? is it just by sheer curiosity, for educational purpose or for a professionnal commitment ?
