How to position a nested DIV at bottom of parent DIV (same as background-position: bottom)

Hi,

I am looking for solution to position a nested DIV at bottom of the parent DIV.
The functionality would be the same as positioning an image using 'background-position: bottom center'

Is this possible using CSS?
I am quite proficient at CSS but cannot think of how this could be achieved.

Would love to hear any input or comments on this.

Cheers.
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<div id="ParentContainer" style="width:200px;height;500px;">
    
   <div id="AnyOldDiv_UsuallyLeftMenu" />
 
    <!-- Need this container to be at bottom of 'ParentContainer' -->
    <div id="NestedContainer" />
 
</div>