Is it possible to make a bookmarklet that creates an alert when the website changes?

I recently found out what a bookmarklet was and I was instantly interested. I was wondering if there was a way to make one that alerts when the website changes (When there's a new unread or new topic in a discourse forum or when I get an email on https://mail.google.com/). I can't use notifications. Here's what I tried:

javascript:(function(){ var xmlHttp = getXMLHttpObj(); if(xmlHttp == null){ alert('Failed to load XMLHTTP'); return; } var actual = getPageContent(xmlHttp, window.location.href); var intervalId = window.setInterval(function(){ var current = getPageContent(xmlHttp, window.location.href); if(actual != current){ alert('This page has been modified since you opened it'); window.clearInterval(intervalId); } }, 1000); function getPageContent(xmlHttp, url){ xmlHttp.open('GET', window.location.href, false); xmlHttp.send(''); return xmlHttp.responseText; } function getXMLHttpObj(){ if(typeof(XMLHttpRequest)!='undefined') return new XMLHttpRequest(); var axO=['Msxml2.XMLHTTP.6.0', 'Msxml2.XMLHTTP.4.0', 'Msxml2.XMLHTTP.3.0', 'Msxml2.XMLHTTP', 'Microsoft.XMLHTTP']; for(var i = 0; i < axO.length; i++){ try{ return new ActiveXObject(axO[i]); }catch(e){} } return null; }})()

Is it possible, and if so how? Also, is changing the favicon possible instead of an alert?