title: The li :after do not work
body: <p>I use the <code>li:after</code> to add content after the <code>li</code>, but not work.</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>body {
    margin: 0;
    padding: 0;
  }
  
  #banner {
    height: 30px;
    width: 750px;
    margin: 0 auto;
    background-color: aquamarine;
  }
  
  #banner ul {
    list-style: none;
    height: 30px;
    padding: 0;
    display: flex;
    align-items: center;
  }
  
  #banner ul li {
    float: left;
    margin: auto 0;
    height: 30px;
    line-height: 30px;
    width: 100px;
    text-align: center;
  }
  
  #banner ul li :after {
    content: "a";
    top: 1px;
    bottom: 1px;
    right: 0;
    width: 5px;
    background-color: black;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;!DOCTYPE html&gt;
  &lt;html lang="en"&gt;
  
  &lt;head&gt;
    &lt;meta charset="UTF-8"&gt;
    &lt;title&gt;title&lt;/title&gt;
    &lt;link rel="stylesheet" href="styles/index.css" type="text/css"&gt;
  &lt;/head&gt;
  
  &lt;body&gt;
    &lt;div id="banner"&gt;
      &lt;ul&gt;
        &lt;li&gt;Home&lt;/li&gt;
        &lt;li&gt;link&lt;/li&gt;
        &lt;li&gt;product&lt;/li&gt;
        &lt;li&gt;phone&lt;/li&gt;
        &lt;li&gt;cat&lt;/li&gt;
        &lt;li&gt;about&lt;/li&gt;
      &lt;/ul&gt;
    &lt;/div&gt;
  
  &lt;/body&gt;
  
  &lt;/html&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>Why the contents do not shows ?</p>