Adds browser specific prefixes and returns resulting css.
    w2utils.cssPrefix(prop, val, returnString)
    
    
        | prop | string, css property | 
    
        | val | string, property value | 
    
        | returnString | boolean, return as string instead of object | 
    
 
This method is used internally in grid, popup, etc. But you can use it as well. It will add prefixes "-webkit", "-moz", "-ms", "-o" and return resulting object.