title: Determine if element property is using % percentage units
body: <p>I'm working on a small framework-ish thing that can make a fixed design fluid.
  So as the window resizes despite the resolution the content fits.</p>
  
  <p>I've had good success converting computed units between px,em &amp; pt. But in most cases getComputedStyle returns the px value of an element conformed to percentages. </p>
  
  <p>How can I obtain the percentage unit defined "by" the CSS efficiently?
  (Thinking ahead I'll need to differentiate between font % and the box model %). </p>
  
  <p>(Please no lodash or jQuery answers,needs to be pure JS) </p>