The tutorial said:

Geometry properties are calculated only for displayed elements.

If an element (or any of its ancestors) has display:none or is not in the document, then all geometry properties are zero (or null for offsetParent).

For example, offsetParent is null, and offsetWidth, offsetHeight are 0 when we created an element, but haven’t inserted it into the document yet, or it (or it’s ancestor) has display:none.

We can use this to check if an element is hidden, like this:

function isHidden(elem) {
  return !elem.offsetWidth && !elem.offsetHeight;
}

But, what if the hidden element is hidden by setting the width to 0, not the height? 😀