title: body.scrollTop vs documentElement.scrollTop vs window.pageYOffset vs window.scrollY
body: <p>When trying to find out how much a web page has been scrolled from the top, which of these should one use: </p>
  
  <p><code>document.body.scrollTop</code>, </p>
  
  <p><code>document.documentElement.scrollTop</code>, </p>
  
  <p><code>window.pageYOffset</code>, </p>
  
  <p><code>window.scrollY</code></p>
  
  <p>Which one(s) would I choose in these 2 separate scenarios:</p>
  
  <p>a) If I wanted maximum compatibility (across the main browsers used currently)?</p>
  
  <p>b) If I wanted code that was most standards compliant/future-proof/strict-mode-compatible (but didn't care about supporting old/non-standard browsers)?</p>