How can I call GetStyle() of nsIDOMNSHTMLElement

Hi,

If I have a pointer to nsIDOMNSHTMLElement* element, how can I call
the GetStyle()?

In nsGenericHTMLElement.h, it said
// nsIDOMNSHTMLElement methods. Note that these are non-virtual
  // methods, implementations are expected to forward calls to these
  // methods.
  // Forward to GetStyle which is protected in the super-class
  inline nsresult GetStyle(nsIDOMCSSStyleDeclaration** aStyle)

But when I do
nsIDOMNSHTMLElement* element;
 nsCOMPtr<nsIDOMCSSStyleDeclaration> cssDecl;

  nsresult result  = element->GetStyle(getter_AddRefs(cssDecl));

My code will not compile.

So can I safely cast my nsIDOMNSHTMLElement* to nsGenericHTMLElement*
and call GetStyle() that way?

Thank you.