On html page load, process something on server side and send a result back to html page

Question 1

Here is how I would do what I think you are explaining. Your question was not very clear.

http://encosia.com/using-jquery-to-directly-call-aspnet-ajax-page-methods/

    $.ajax({
      type: "POST",
      url: "PageName.aspx/MethodName",
      data: "{}",
      contentType: "application/json; charset=utf-8",
      dataType: "json",
      success: function(msg) {
        // Do something interesting here.
      }
    });

HTML

    <html>
     <head>
      <title>Calling a page method with jQuery</title>
      <script type="text/javascript" src="jquery-1.2.6.min.js"></script>
      <script type="text/javascript" src="Default.js"></script>
    </head>
    <body>
      <div id="Result">Click here for the time.</div>
    </body>
    </html>

JS

    $(document).ready(function() {
      // Add the page method call as an onclick handler for the div.
      $("#Result").click(function() {
        $.ajax({
          type: "POST",
          url: "Default.aspx/GetDate",
          data: "{}",
          contentType: "application/json; charset=utf-8",
          dataType: "json",
          success: function(msg) {
            // Replace the div's content with the page method's return.
            $("#Result").text(msg.d);
          }
        });
      });
    });

But again I really have no idea what you are trying to do. You were really vague.

Question 2

Yes, simpler approach is ajax.

This is pseudo javascript code...

window.onload = function(){
    ajax.open("your server url");
    ajax.on("success",function(){
        alert("okay, done.");
    });
};

Question 3

Using iFrame for the process you have requested, is not recommended.

Consider including AJAX to route your request to a server resource. In your case it should be an asp page.

Here's a beginners guide to using ajax with asp http://www.w3schools.com/asp/asp_ajax_asp.asp

PS: When using AJAX make sure you are aware about

Cross Domain Requests http://msdn.microsoft.com/en-us/library/dd573303%28v=vs.85%29.aspx
Access-Control-Allow-Origin

Here's a pseudo code for you to work on:

<script>
function funcName(){
var xmlhttp;
if (window.XMLHttpRequest)
 {// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
 document.getElementById("<yourOwnElement'sID>").innerHTML=xmlhttp.responseText;
}
}
xmlhttp.open("GET","serverResource.asp?q="+<parameters>,true);
xmlhttp.send();
}
</script>