Edit in JSFiddle

Try turning off your Internet connection.

* {
    font-family: sans-serif;
    font-weight: bold;
}

function uOffline() {
    document.body.innerHTML =
        'See, I saw you do that.'
        + '<br /><br />'
        + 'navigator.onLine === '+ navigator.onLine;
}
function uOnline() {
    document.body.innerHTML =
        '&hellip;and, we\'re back!'
        + '<br /><br />'
        + 'navigator.onLine === '+ navigator.onLine;
}

if(window.addEventListener) {
    window.addEventListener('offline', uOffline);
    window.addEventListener('online', uOnline);
} else {
    document.body.attachEvent('onoffline', uOffline);
    document.body.attachEvent('ononline', uOnline);
}