title: css "left" not working
body: <p>I have 2 divs, parent and child, I want that child left side (left border) will in center of parent.</p>
  
  <p>Why this code not working? that is <code>left: 50%</code> for child, is not working.</p>
  
  <pre><code>&lt;div id="outher"&gt;
      &lt;div id="inner"&gt;
  
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>css:</p>
  
  <pre><code>#outher {
     width: 1000px;
     height: 1000px;
     background-color: #ccc;
  }
  
  #inner {
     width: 400px;
     height: 300px;
     background-color: #090;
     left: 50%;
  }
  </code></pre>
  
  <p>demo <a href="http://jsfiddle.net/vrse2/5/">http://jsfiddle.net/vrse2/5/</a></p>