REM example

@import url(http://fonts.googleapis.com/css?family=Open+Sans:300,400,700);
$base-font-family: 'Open Sans', sans-serif;
$background-color: #fff;

@import 'media-queries';

// Set a rem font size with pixel fallback
@function calculate-rem($size) {
  $rem-size: $size / 16px;
  @return $rem-size * 1rem;
}

@mixin font-size($size) {
  font-size: $size;
  font-size: calculate-rem($size);
}


html {
  font-size: 10px;
  width: 100%;
  height: 100%;
  @include min-breakpoint(screen-xs) {
    font-size: 12px;
  }
  @include min-breakpoint(screen-sm) {
    font-size: 14px;
  }
  @include min-breakpoint(screen-md) {
    font-size: 16px;
  }
}

body {
  width: 100%;
  height: 100%;
  margin: 0;
  padding: 0;
  font-family: $base-font-family;
  line-height: 1.2em;
  color: #333;
  background: #fff;
}

header {
  background: #111;
}

.headline {
  position: relative;
  height: 400px;
  color: #fff;
  text-align: center;
  background: url('https://farm9.staticflickr.com/8088/8363737128_b76782c7a2_b.jpg') no-repeat center center;
  background-size: cover;
  &:after {
    position: absolute;
    content: '';
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(#111, 0.5);
  }
}

.inner {
  position: absolute;
  top: 50%;
  left: 50%;
  opacity: 0;
  z-index: 10;
  transform: translateX(-50%) translateY(-50%);
  animation: fade-in 0.75s 0.25s ease-in forwards;
  h1 {
    @include font-size(64px);
    margin: 0;
    line-height: 1.2em;
  }
  p {
    @include font-size(22px);
    margin: 0;
    font-style: italic;
    font-family: serif;
  }
}

nav {
  position: fixed;
  width: 100%;
  z-index: 10;
  &.scrolled {
    background: rgba(#111, 0.9);
  }
  ul {
    float: right;
    list-style: none;
    padding: 25px;
    margin: 0;
  }
  li {
    float: left;
  }
  a {
    @include font-size(14px);
    color: #fff;
    text-decoration: none;
    margin: 5px 0 0 20px;
    display: block;
    &:hover {
      color: darken(#fff, 7%);
    }
  }
}

.logo {
  height: 75px;
  float: left;
  &:before, &:after {
    @include font-size(12px);
    position: absolute;
    font-weight: 300;
    line-height: 0;
    color: #fff;
    top: 25px;
  }
  &:before {
    content: '\00BB';
    left: 20px;
  }
  &:after {
    content: '\00AB';
    left: 34px;
  }
}

section {
  @include font-size(18px);
  width: 100%;
  margin: 25px auto;
  padding: 20px;
  max-width: 800px;
  line-height: 1.6em;
  box-sizing: border-box;
}

@keyframes fade-in {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}