I found this on JavaScriptKit. It shows a nice way of including html on an already loaded page.

function HttpRequest(url){
var pageRequest = false //variable to hold ajax object
/*@cc_on
   @if (@_jscript_version >= 5)
      try {
      pageRequest = new ActiveXObject("Msxml2.XMLHTTP")
      }
      catch (e){
         try {
         pageRequest = new ActiveXObject("Microsoft.XMLHTTP")
         }
         catch (e2){
         pageRequest = false
         }
      }
   @end
@*/

if (!pageRequest && typeof XMLHttpRequest != 'undefined')
   pageRequest = new XMLHttpRequest()

if (pageRequest){ //if pageRequest is not false
   pageRequest.open('GET', url, false) //get page synchronously
   pageRequest.send(null)
   embedpage(pageRequest)
   }
}

function embedpage(request){
//if viewing page offline or the document was successfully retrieved online (status code=2000)
if (window.location.href.indexOf("http")==-1 || request.status==200)
   document.write(request.responseText)
}

HttpRequest("external.htm") //include "external.htm" onto current page

This is true if you decide to return the complete rendered markup, most XHR implementations I have done either return JSON or XML... and sometimes just a js function call, then this isn't a problem.

If you want to return the complete rendered page, you have more things to take care of besides the form element... the html/head/body and so on are all elements that should only exist one time for a page to be considered valid (among other things)…