<p style="margin-top:0;">
<asp:TextBox ID="txtContent" runat="server" TextMode="MultiLine" Height="450" Width="98%"></asp:TextBox>
</p>

try { stringWidth("x"); }
catch (e) {
if (e.message.match(/runtime/i) || e.message.match(/运行时/i) )
e = new Error("A CodeMirror inside a P-style element does not work in Internet Explorer. (innerHTML bug)");
throw e;
}

<div style="margin-top:0;">
<asp:TextBox ID="txtContent" runat="server" TextMode="MultiLine" Height="450" Width="98%"></asp:TextBox>
</div>