title: Why margin:0 auto can't center image in div levelly?
body: <p>The image element can be centered levelly with <code>margin:0px 50px 0px 50px;</code>.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.wrapper {
    width: 175px;
    height: 100px;
    border: 1px solid red;
  }
  
  img {
    margin: 0px 50px 0px 50px;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="wrapper"&gt;
    &lt;img src="http://i.imgur.com/9OIT14w.jpg" alt=""&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>In the situation, <code>margin:0 auto; ==  margin:0px 50px 0px 50px;</code>.<br>
  So it is equal to write <code>margin:0px 50px 0px 50px;</code> as <code>margin:0 auto;</code>.<br>
  Why it can't be centered with <code>margin:0 auto;</code>?  </p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.wrapper {
    width: 175px;
    height: 100px;
    border: 1px solid red;
  }
  
  img {
    margin: 0 auto;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="wrapper"&gt;
    &lt;img src="http://i.imgur.com/9OIT14w.jpg" alt=""&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>