Don't use fixed css units like px and pt. Use em or ex.
em 1em is equal to the current font size. 2em means 2 times the size of the current font. E.g., if an element is displayed with a font of 12 pt, then '2em' is 24 pt. The 'em' is a very useful unit in CSS, since it can adapt automatically to the font that the reader uses ex one ex is the x-height of a font (x-height is usually about half the font-size)