CSS Units Tutorial

an em is equal to the size of the font that applies to the parent of the element

However, em values compound. Nested elements will have their em values multiplied by their parent's em value.

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!