Typography

The rem unit is relative to the font-size of the root element html . That means that we can define a single font size on the root element, and define all rem units to be a percentage of that.
Font:

body
  font-family 'Avenir', Helvetica, Arial, sans-serif
  font-weight normal
  -webkit-font-smoothing antialiased
  -moz-osx-font-smoothing grayscale
Font Size:

html
  font-size 18px
Titles:

Heading h1

2.7 rem 50 px

Heading h2

2.2 rem 40 px

Heading h3

1.6 rem 30 px

Heading h4

1.3 rem 24 px

Body

1 rem 18 px

Small

.8 rem 16 px

Extra small

.7 rem 14 px
Example:
  
<h1 class="ui-title-1"> Heading h1 </h1>
<h2 class="ui-title-2"> Heading h2 </h2>
<h3 class="ui-title-3"> Heading h3 </h3>
<h4 class="ui-title-4"> Heading h4 </h4>
<p class="ui-text-regular"> Body </p>
<p class="ui-text-small"> Small </p>
<p class="ui-text-smaller"> Extra small </p>