Check out example codes for "css height auto vs 100". It will help you in understanding the concepts better.

Code Example 1

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

Code Example 2

<div style="height:50px">
    <div id="innerDiv" style="height:100%">
    </div>
</div>

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