How do I make a div fill its parent element vertically?

Question

I'm trying to get #sidebar to fill #main vertically (and match #content's height). How do I do this?

Here's my jsFiddle.

CSS:

#main {
   border: solid green 2px;
}

#sidebar {
    background-color: red;
    width: 20%;
    overflow: auto;
    float: left;
    height: 100%;
}

#content {
    background-color: orange;
    width: 80%;
    overflow: auto;
    float: left;
    height: 100%;
}

#main-footer {
    clear: both;
}​

And html:

<div id="main">

 <div id="sidebar">
     <ul>
         <li>abc</li>
         <li>def</li>
     </ul>
 </div>

 <div id="content">
defsdfasdfasd fjasd;lf jasdl;fkjasd; lfjas; lfkjadsfl;sdajfs dlfjsdlfksjflaskfjsdaljfasdl asdfsad sfljsf sadfjsldfj sakfj;alsdfj dfsjsadfl;j asdlfjl;asdfj 
defsdfasdfasd fjasd;lf jasdl;fkjasd; lfjas; lfkjadsfl;sdajfs dlfjsdlfksjflaskfjsdaljfasdl asdfsad sfljsf sadfjsldfj sakfj;alsdfj dfsjsadfl;j asdlfjl;asdfj 
 </div>

 <div id="main-footer">
 </div>

</div>​

Answer 1

Without setting the div height explicitly it is a little tricky. Here is one solution.

Answer 2

div is a container. It will take up the height its contents take up. The only way to acheive what you want is to use height in px. If you do not care about semantic markup(I recommend you should) then septemberbrain's link is a good trick.