Freecodecamp: 简单的 css 问题 .thick-green-border 类未通过。 我究竟做错了什么？

挑战在元素周围添加边框有问题。
用户代理是： Mozilla/5.0 (Windows NT 6.1; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0 。
请描述如何重现此问题，并尽可能提供屏幕截图的链接。

我的代码：

<link href="https://fonts.googleapis.com/css?family=Lobster" rel="stylesheet" type="text/css">
<style>
  .red-text {
    color: red;
  }

  h2 {
    font-family: Lobster, Monospace;
  }

  p {
    font-size: 16px;
    font-family: Monospace;
  }

  .smaller-image {
    width: 100px;
  }

  .thick-green-border {
    border-width: 10px;
    border-style: solid;
    border-color: green;
  }
</style>

<h2 class="red-text">CatPhotoApp</h2>

<img class="smaller-image" class="thick-green-border"  src="https://bit.ly/fcc-relaxing-cat">

<p class="red-text">Kitty ipsum dolor sit amet, shed everywhere shed everywhere stretching attack your ankles chase the red dot, hairball run catnip eat the grass sniff.</p>
<p class="red-text">Purr jump eat the grass rip the couch scratched sunbathe, shed everywhere rip the couch sleep in the sink fluffy fur catnip scratched.</p>