The second way to implement a responsive font size is to use media queries along with fixed-size text, using ems, rems and px values: px values specify the height of the letters in CSS pixels em is relative to parent's font size, eg. 2em = 28px if parent element's font-size is 14px. rem (root em) is relative to the html element's font-size. 1 rem measures the same through the whole document. ems get hard to maintain in rules nested too deeply, but rems have no problem. The best way to use ems and pixels is to specify a pixel font-size for the html element and use ems and rems for the rest. There are several reasons to do it this way: