You can try asking them if they want to visit through window.prompt:
if(window.prompt('Do you wish to visit the following website?','http://www.google.ca')) location.href='http://www.google.ca/';
In addition, Internet Explorer supports modal dialogs, so you can try to show one of them:
if (window.showModalDialog) window.showModalDialog("mypage.html","popup","dialogWidth:255px;dialogHeight:250px"); else window.open("mypage.html","name","height=255,width=250,toolbar=no,directories=no,status=no,menubar=no,scrollbars=no,resizable=no,modal=yes");
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