css - How to make a floated div 100% height of its parent?

Question

Here is the HTML:

<div id="outer">
    <div id="inner"></div>
    Test
</div>

And here is the CSS:

#inner {
  float: left;
  height: 100%;
}

Upon inspection with the Chrome developer tools, the inner div is getting a height of 0px.

How can I force it to be 100% of the height of the parent div?

Answer 1

For the parent:

display: flex;

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

Edit: Only drawback is IE as usual, IE9 does not support flex. http://caniuse.com/flexbox

Edit 2: As @toddsby noted, align items is for parent, and its default value actually is stretch. If you want a different value for child, there is align-self property.

Edit 3: jsFiddle: https://jsfiddle.net/bv71tms5/2/