How to force child div to be 100% of parent div's height without specifying parent's height?

Question

I have a site with the following structure:

  

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Answer 1

The easiest way to do this is to just fake it. A List Apart has covered this extensively over the years, like in this article from Dan Cederholm from 2004.

Here's how I usually do it:

You can easily add a header onto this design by wrapping #container in another div, embedding the header div as #container's sibling, and moving the margin and width styles to the parent container. Also, the CSS should be moved into a separate file and not kept inline, etc. etc. Finally, the clearfix class can be found on positioniseverything.