Depends what you mean by "post data". You can use the HTML target="" attribute on a <form /> tag, so it could be as simple as:
<form action="do_stuff.aspx" method="post" target="my_iframe">
<input type="submit" value="Do Stuff!">
</form>
<!-- when the form is submitted, the server response will appear in this iframe -->
<iframe name="my_iframe" src="not_submitted_yet.aspx"></iframe>
If that's not it, or you're after something more complex, please edit your question to include more detail.
There is a known bug with Internet Explorer that only occurs when you're dynamically creating your iframes, etc. using Javascript (there's a work-around here), but if you're using ordinary HTML markup, you're fine. The target attribute and frame names isn't some clever ninja hack; although it was deprecated (and therefore won't validate) in HTML 4 Strict or XHTML 1 Strict, it's been part of HTML since 3.2, it's formally part of HTML5, and it works in just about every browser since Netscape 3.
I have verified this behaviour as working with XHTML 1 Strict, XHTML 1 Transitional, HTML 4 Strict and in "quirks mode" with no DOCTYPE specified, and it works in all cases using Internet Explorer 7.0.5730.13. My test case consist of two files, using classic ASP on IIS 6; they're reproduced here in full so you can verify this behaviour for yourself.
default.asp
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>
<title>Form Iframe Demo</title>
</head>
<body>
<form action="do_stuff.asp" method="post" target="my_frame">
<input type="text" name="someText" value="Some Text">
<input type="submit">
</form>
<iframe name="my_frame" src="do_stuff.asp">
</iframe>
</body>
</html>
do_stuff.asp
<%@Language="JScript"%><?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>
<title>Form Iframe Demo</title>
</head>
<body>
<% if (Request.Form.Count) { %>
You typed: <%=Request.Form("someText").Item%>
<% } else { %>
(not submitted)
<% } %>
</body>
</html>
I would be very interested to hear of any browser that doesn't run these examples correctly.
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