css height auto vs 100

CSS
<div style="height:50px">
    <div id="innerDiv" style="height:auto">
          <div id="evenInner" style="height:10px">
          </div>
    </div>
</div><div style="height:50px">
    <div id="innerDiv" style="height:100%">
    </div>
</div>
Source

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