HTML5 Center

I have a page with a panel within a panel. I want the inside iframe to be a specific width but to be center aligned on the page. I have tried numerous things, but below is my latest attempt. However, the panel displays flush left on the screen.

<div style="text-align:center;width:100%;">
<asp:Panel ID="panIframe" style="position: relative;width:835px; height:542px;" runat="server">
<iframe id="SCO" name="" style="width: 100%; height: 100%;"></iframe>
</asp:Panel>
</div>

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Truth of the matter is, the below WORKS! But gives me squiggle messages in VS2012 that <center> is not supported in HTML5.

<center>
<asp:Panel ID="panIframe" style="width:835px; height:542px;" runat="server">
<iframe id="SCO" name="" style="width: 100%; height: 100%;"></iframe>
</asp:Panel>
</center>

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How do I get the panel to center on the width of the page and be supported in HTML5?

thanks!