<!DOCTYPE html>
<html lang="en-US">
<body offline="offFunction()" ononline="onFunction()">
<p>Open Firefox, Press F10 and choose File > work offline mode</p>
<script>
function offFunction(){
alert("You are working in offline mode");
}
function onFunction(){
alert("You are working in online mode");
}
</script> </body>
</html>