摘自w3c em unit Equal to the computed value of the ‘font-size’ PRoperty of the element on which it is used.

means that the line height of h1 elements will be 20% greater than the font size of h1 element. On the other hand:

means that the font size of h1 elements will be 20% greater than the computed font size inherited by h1 elements.

rem unit Equal to the computed value of ‘font-size’ on the root element.

When specified on the ‘font-size’ property of the root element, the ‘rem’ units refer to the property’s initial value.