title: how calculate top property for fixed element
body: <p>:)
  when i dont set top/left properties for an element fixed what acccured??
  please see this sample code:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>#fixed-menu{
      background-color:#ba4444;
      border-top: 5px solid #0892cd;
      height: 60px;
      position: fixed;
      width: 100%;
      z-index:9999;
      box-shadow:rgb(128, 128, 128) 0px 5px 15px 0px;
  }
  #wrapper{
      height:900px;
      width:960px;
      margin:0 auto;
      background-color:yellow;
      margin-top:100px;
    
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;body&gt;
    &lt;div id="fixed-menu"&gt;&lt;/div&gt;
    &lt;div id="wrapper"&gt;&lt;/div&gt;
  &lt;body&gt;</code></pre>
  </div>
  </div>
  
  with above code,fixed-menu also have 100px margin-top!!!!why?????
  ...................
  how calculated top property???</p>