Problem is - where do you get the HtmlDocument from? You can only get it from the WebBrowser control, which implies using both controls at once. In which case there's no real benefit in loading an HtmlDocument; you might as well grab the HTML source and load that.
That is not always the truth. I get my document from CreateDocumentFromUrl wich has noting to do with the webbrowser control. And I am about to go insane over the webbrowser control and the lag of showing a htmldocument. So I found the HtmlEditor intersting.