Generated by SassMeister.com.

SassMeister-input-HTML.html

<p class="box">Default: Border Box</p>
<div class="component">
  <p class="box">Component: Content Box</p>  
</div>

SassMeister-input.scss

// ----
// Sass (v3.3.10)
// Compass (v1.0.0.alpha.20)
// Susy (v2.1.2)
// ----

@import "susy";
html {
  box-sizing: border-box;
}

*, *:before, *:after {
  box-sizing: inherit;
}

// uncomment this to see why Susy currently
// does not work
// @include global-box-sizing(border-box);

.component {
  box-sizing: content-box;
}

.box {
  background-color: #aaa;
  border: 1px solid #000;
  width: 10rem;
  height: 3rem;
  padding: 0.5rem;
  margin: 0.5rem;
}

SassMeister-output.css

html {
  box-sizing: border-box;
}

*, *:before, *:after {
  box-sizing: inherit;
}

.component {
  box-sizing: content-box;
}

.box {
  background-color: #aaa;
  border: 1px solid #000;
  width: 10rem;
  height: 3rem;
  padding: 0.5rem;
  margin: 0.5rem;
}

SassMeister-rendered.html

<p class="box">Default: Border Box</p>
<div class="component">
  <p class="box">Component: Content Box</p>  
</div>