Untitled

dabblet.css

html {
  /* Commenting out this stops the red div from filling vertically. WTF? */
  height: 100%;
}

body {
  height: 100%;
  background-color: blue;
}

div.red {
  height: 100%;
  background-color: #600;
}

div.green {
  background-color: green;
  height: 100px;
}

dabblet.html

<div class='red'>
  <div class='green'>&nbsp;</div>
</div>

settings.json

{"view":"split","fontsize":"100","seethrough":"","prefixfree":"1","page":"css"}