Responsive Typography

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Resize the base font-size accordingly to the viewport size. Start with 1.6rem and reach a max of 2.2rem.
/* get an easy math for rem units */
html { font-size: 62.5%; }
body { font-size: 1.6rem; }

/* where the magic happens */ 

@media (min-width: 320px) {
  body {
    font-size: calc(1.6rem + 6 * ((100vw - 320px) / 680));
  }  
}

@media (min-width: 1000px) {
  body {
   font-size: 2.2rem;
  }
}
Read more about fluid typography, and take a look at this codepen.