Bei mir läuft das so auch ganz gut.
Delphi-Quellcode:
If WebBrowser1.Document = nil Then WebBrowser1.Navigate('about:blank');
OleVariant(WebBrowser1.Document as IHTMLDocument2).Clear;
OleVariant(WebBrowser1.Document as IHTMLDocument2).Write(DeinString);
OleVariant(WebBrowser1.Document as IHTMLDocument2).Close;