Indication while site is loaded in IFRAME

On my ASPX page I am showing other webiste in a Iframe, in fact I am posting a form to other domain and target of that form is Iframe. It takes little while by the time page is shown in Iframe. So I would like to show some indication that page is loading and take it out when page ( From other domain) is loaded in Inframe. How do I do that? I tried onload event of the IFrmae but it doesn't work exactly the message goes away just before loading the page. Thanks!

THis is my code. On button click event I post the following form ( Action is on the other domain)

<form id="frmSSO" action='<%=formAction%>' method="post" name="frmSSO" targe开发者_JAVA技巧t='iframe1' onload="hideProgress()" >
    <input type="hidden" name="TARGET" value='<% = formTarget%>' />
    <input type="hidden" name="Encoding" value="B64" />
    <input type="hidden" name="SAMLResponse" value='<% = resp%>' />              
</form>

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You can layer a div over top of the iframe temporarily, but there is no way to know when it is safe to hide the div.