When length of an element is specified using em,
it is calculated based on the font size
of the element.
If an element has no font-size specified,
the length is calculated based on the
font size of its nearest parent element.
In the above example, the width and height
of the h1 were calculated based on the
font-size of its parent element (div),
as font-size was not specified
for the child element (h1).