If I'm understanding you correctly, you want a page with two iframes, one of which should refresh periodically, taking data from a form on the top page. I've no idea what "an (content) extension" is, so I'm going to ignore it for now.
I think the way you're approaching this is fundamentally wrong. It may be that I'm just failing to grasp your design, but it seems to me that it would be a much better idea to only refresh the iframe when the form changes. For example:
Code:
<form action="a.aspx" target="iframe_a" name="gallery_form">
<select onchange="this.form.submit();">
<option value="1">Gallery One</option>
<option value="2">Gallery Two</option>
<option value="3">Gallery Three</option>
</select>
<input type="submit" value="Go" name="submit_button">
</form>
<script type="text/javascript">
// Hide the submit button for Javascript-capable browsers.
document.forms['gallery_form'].elements['submit_button'].style.display = "none";
</script>
<iframe name="iframe_a">
<!--
Alternative content not required, since frame-incapable browsers
will simply display the results of the submission in the main window.
-->
</iframe>
However, it would be a saner idea, in my opinion, to use a single static page without the iframes, and links for the galleries. This would provide the maximum accessibility, and keep vital browser functions (like "back" and "bookmark") working the way they should, as well as conforming to Strict DOCTYPEs.
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