window.getSelection() gives me the selected text, but I want the HTML

I'm extending a WYSIWYG HTML editor (for Firefox), I want to add tags around a selection. I can't find a function to accomplish this in the Mozilla Midas specification.

There is a command to replace the selection with HTML.
So if I could read the selection contents, I could add my tags to this string.

window.getSelection() almost works, but it gives me nsISelection which converts to a plain-text string.

PS: document.getSelection() returns the plain-text string not even a nsISelection.