title: Setting CSS top percent not working as expected
body: <p>I tried a responsive css layout,but "top:50%" can't work and the "left:50%" can.
  Why it happened?</p>
  
  <pre><code>&lt;div style="position:relative;top:0%;left:0%;height:100%;width:100%"&gt;
      &lt;div style="position:absolute;top:50%;left:50%;"&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>