There are several ways to achieve this.

*CSS supports dimensions that are relative to viewport.

3.2vw = 3.2% of width of viewport

3.2vh = 3.2% of height of viewport

3.2vmin = Smaller of 3.2vw or 3.2vh

3.2vmax = Bigger of 3.2vw or 3.2vh

body {
  font-size: 3.2vw;
}

see http://css-tricks.com/viewport-sized-typography/
and also look at http://caniuse.com/viewport-units