title: Element's coordinates relative to its parent
body: <p>The method <code>el.getBoundingClientRect()</code> gives a result relative to the viewport's top-left corner (<code>0,0</code>), not relative to an element's parent, whereas <code>el.offsetTop</code>, <code>el.offsetLeft</code> (etc.) give a result relative to the parent.</p>
  
  <p>What is the best practice to have the coordinates of an element relative to its parent? <code>el.getBoundingClientRect()</code> modified (how?) to use parent as <code>(0,0)</code> coordinate, or still <code>el.offsetTop</code>, <code>el.offsetLeft</code> and so on?</p>