How can i get the background-image url of an  element node? 

Usually, I can read the "background-image" attribute of the node. But if it is written 
in the style sheet, there is no explicit "background-image" attribute in the element's 
attributes list. Instead, there is a "class" attribute.

I have also tried to get some information from RenderStyle. But it can only provide an
interface hasBackgroundImage() .  

How can I get it? It seems not to be an elegant way that we firstly read the style class
abtribute and then parse the style sheet file. Could WebKit provides an expandedAttributes()
interface which can return all the attributes by simply expanding the class attribute with 
those written in the style sheet?