The Javascript use in the page : if (navigator.appName == "Netscape" || (navigator.appName == "Microsoft Internet Explorer" && navigator.appVersion.substring(17, 25) == "MSIE 8.0")) { $get('<%= Panel1.ClientID %>').style.width = '600px'; $get('<%= Panel2.ClientID %>').style.width = '600px'; $get('<%= Panel3.ClientID %>').style.width = '600px'; $get('<%= Map1.ClientID %>').style.width = '600px'; } else { $get('<%= Panel1.ClientID %>').style.width = '100%'; $get('<%= Panel2.ClientID %>').style.width = '100%'; $get('<%= Panel3.ClientID %>').style.width = '100%'; $get('<%= Map1.ClientID %>').style.width = '100%'; } The declaration of the element : The HTML generate :