CSS DIV Height in Percent

Hi Experts.

How can I make an inner DIV span to a 100% height?

Example, http://codepen.io/AleksPoposki/pen/Opovpv?editors=1100

You will see that I've  set up #outer to grow, in this case according to #inner1. But, how can I make #inner2 the same heigh? Do I need flex?
APD TorontoSoftware DeveloperAsked:
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Elvio Lujan.Net Senior DeveloperCommented:
you need to set the parent height in a fixed size, in your case:

  background-color: red;
  overflow: auto;
  height: 200px;

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