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

Question

For the parent:

display: flex;

You should add some prefixes, http://css-tricks.com/using-flexbox/.

As @Adam Garner noted, align-items: stretch; is not needed. Its usage is also for parent, not children. If you want to define children stretching, you use align-self.

.parent {
  background: red;
  padding: 10px;
  display:flex;
}

.other-child {
  width: 100px;
  background: yellow;
  height: 150px;
  padding: .5rem;
}

.child {  
  width: 100px;
  background: blue;
}

<div class="parent">
  <div class="other-child">
    Only used for stretching the parent
  </div>
  <div class="child"></div>
</div>

NOTE: This answer is applicable to legacy browsers without support for the Flexbox standard. For a modern approach, see: https://stackoverflow.com/a/23300532/1155721

---

I suggest you take a look at Equal Height Columns with Cross-Browser CSS and No Hacks.

Basically, doing this with CSS in a browser compatible way is not trivial (but trivial with tables) so find yourself an appropriate pre-packaged solution.

Also, the answer varies on whether you want 100% height or equal height. Usually it's equal height. If it's 100% height the answer is slightly different.

This is a frustrating issue that's dealt with designers all the time. The trick is that you need to set the height to 100% on BODY and HTML in your CSS.

html,body {
    height:100%;
}

This seemingly pointless code is to define to the browser what 100% means. Frustrating, yes, but is the simplest way.

I find that setting the two columns to display: table-cell; instead of float: left; works well.

If you don't mind the navigation div being clipped in the event of an unexpectedly-short content div, there's at least one easy way:

#main {
position: relative;
}

#main #navigation {
position: absolute;
top: 0;
left: 0;
bottom: 0;
width: 10em; /* or whatever */
}

#main #content {
margin: 0;
margin-left: 10em; /* or whatever width you set for #navigation */
}

Elsewise there's the faux-columns technique.

#main {
    overflow: hidden;
}
#navigation, #content {
    margin-bottom: -1000px;
    padding-bottom: 1000px;
}

using jQuery:

$(function() {
    function unifyHeights() {
        var maxHeight = 0;
        $('#container').children('#navigation, #content').each(function() {
            var height = $(this).outerHeight();
            // alert(height);
            if ( height > maxHeight ) {
                maxHeight = height;
            }
        });
        $('#navigation, #content').css('height', maxHeight);
    }
    unifyHeights();
});