You really should know what a libraries functions are before you use it. Many times in jQuery they are wrappers to existing which ALSO compensate for how browsers interpret various commands.
You should be able to lookup all of the methods you have in that block, and while you are there read up on what may be different if you use IE6 as opposed to chrome.
Here is a quick start for you:
document.addEventListener("DOMContentLoaded", function(event) {
var aboutme = document.getElementById('about').offsetTop;
var project = document.getElementById('jobs').offsetTop;
var contactme = document.getElementById('contact').offsetTop;
}