Edit in JSFiddle

$(document).ready(function () {
    if (jQuery.support.leadingWhitespace) {
        document.body.innerHTML = '<div class="alert">Upgrade your browser, the web will look a lot better, seriously. :)</div>' + document.body.innerHTML;
    }
});

              
            
          
            
              
                
.alert {
    padding:5px;
    background-color:#bada55;
    color:#252525;
}