title: Detect element currently in viewport
body: <p>I have a page with a fixed header, and I am trying to get the <code>li</code> elements within the nav to change styling based on the user's window based on current scroll point.</p>
  
  <p>I'm under the impression that the best way to do this would be if I worked out what element was currently at the top of the viewport. If I've got 4+ <code>divs</code> all with the class <code>page-section</code>, how do I find the <code>id</code> of the one currently at the top of the page?</p>