As the follow up, here is the source code sample, you can insert an <iframe> tag in your aspx or html page:
<iframe id="ifHtmlContent" style="position:absolute"/>
Then you can use this object from your Silverlight code using the HTML-DOM bridge:
System.Windows.Browser.HtmlElement myFrame = System.Windows.Browser.HtmlPage.Document.GetElementById("ifHtmlContent");
if (myFrame != null)
{
myFrame.SetStyleAttribute("width", "1024");
myFrame.SetStyleAttribute("height", "768");
myFrame.SetAttribute("src", txtURI.Text);
myFrame.SetStyleAttribute("left", "0");
myFrame.SetStyleAttribute("top", "50");
myFrame.SetStyleAttribute("visibility", "visible");
}