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                <div class="container">
  <div class="inner">

    <h1>Viewport Breakpoint Font Sizer</h1>

    <p><a href="" target="_blank">Available on GitHub!</a></p>

    <p>This is a Sass function I wrote to facilitate predictable viewport-based font sizes.</p>

    <p>I wrote it to accompany my conference session on viewport units.</p>

    <p>It uses simple math to deliver the combination of settings that will give us the font size we desire at a specific "threshold" breakpoint above which (and below which) we want the font size to increase at specific (and different) rates.</p>
    <p>That's a bit of a mouthful, but resize your browser (horizontally) and you'll see what I am on about!</p>




                // The maximum size you would like your site container to be.
// Can use any unit, including percents (so you can use 100% if you like).
$max-site-width: 1400px;

// The size you want your font to be at the "switch point".
$threshold-px: 14px;

// The breakpoint at which you want your font size to be $threshold-px.
$switch-point: 850px;

// The rate that you want font size to increase as browser width increases from $switch-point.
$vw-rate-up: 1.5vw;

// The rate that you want font size to increase as browser width decreases from $switch-point.
$vw-rate-down: 1.125vw;

$up-size: viewportFontSizer($threshold-px, $switch-point, $vw-rate-up, 'add');
$down-size: viewportFontSizer($threshold-px, $switch-point, $vw-rate-down, 'subtract');

.container {
  font-size: calc( #{$down-size} - #{$vw-rate-down} );
  max-width: $max-site-width;
  margin: 0 auto;
  background-color: rgba(white, .5);

  @media only screen and (min-width: $switch-point) {
    font-size: calc( #{$up-size} + #{$vw-rate-up} );

  @media only screen and (min-width: $max-site-width) {
    font-size: calc( #{$up-size} + #{$max-site-width/100 * strip-unit($vw-rate-up)} );

// Just the optional styling to make it pretty.
* {
  box-sizing: border-box;

html {
  font-size: 16px;

body {
  background-color: rgba(black, .4);
  color: rgba(black, .75);
  font-family: helvetica, arial, sans-serif;
  font-size: 1em;
  margin: 0;
  padding: 1em;

h1, h2, h3, h4, p {
  margin-top: 0;

h1, h2, h3, h4 {
  margin-bottom: .5em;

p {
  margin-bottom: 1em;
  &:last-child {
    margin-bottom: 0;

.container {
  max-width: $max-site-width;
  margin: 0 auto;
  background-color: rgba(white, .75);

.inner {
  padding: 1em;