CSS Units Tutorial
em is equal to the size of the font that applies to the parent of the element
em values compound. Nested elements will have their
em values multiplied by their parent's
rem values were invented in order to sidestep the compounding problem.
rem values are relative to the root
html element, not the parent element. In other words, it lets you specify a font size in a relative fashion without being affected by the size of the parent, thereby eliminating compounding.
Try experimenting with various css-units for these elements!