Use HTML5 to Require a Field(表单必填项)

Use HTML5 to Require a Field

说明：

当你设计表单时，你可以指定某些选项为必填项(required)，只有当用户填写了该选项后，用户才能够提交表单。

例如，如果你想把一个文本输入字段设置为必填项，在你的input元素中加上required属性就可以了，你可以使用：
<input type="text" required>

任务：给你的输入框添加required属性，这样用户不填写输入框就无法提交表单。

试试输入框不填写任何文本，然后点击Submit提交表单，看看浏览器如何提示你该字段为必填项。

注意：required属性在Safari浏览器中不起作用，请用其他浏览器来学习，推荐使用Chrome。

原始代码：

<link href="https://fonts.gdgdocs.org/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;
  }

  .thick-green-border {
    border-color: green;
    border-width: 10px;
    border-style: solid;
    border-radius: 50%;
  }

  .smaller-image {
    width: 100px;
  }
</style>

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

<p>Click here for <a href="#">cat photos</a>.</p>

<a href="#"><img class="smaller-image thick-green-border" alt="A cute orange cat lying on its back" src="/images/relaxing-cat.jpg"></a>

<p>Things cats love:</p>
<ul>
  <li>cat nip</li>
  <li>laser pointers</li>
  <li>lasagna</li>
</ul>
<p>Top 3 things cats hate:</p>
<ol>
  <li>flea treatment</li>
  <li>thunder</li>
  <li>other cats</li>
</ol>
<form action="/submit-cat-photo">
  <input type="text" placeholder="cat photo URL">
  <button type="submit">Submit</button>
</form>

调试代码：

<link href="https://fonts.gdgdocs.org/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;
  }

  .thick-green-border {
    border-color: green;
    border-width: 10px;
    border-style: solid;
    border-radius: 50%;
  }

  .smaller-image {
    width: 100px;
  }
</style>

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

<p>Click here for <a href="#">cat photos</a>.</p>

<a href="#"><img class="smaller-image thick-green-border" alt="A cute orange cat lying on its back" src="/images/relaxing-cat.jpg"></a>

<p>Things cats love:</p>
<ul>
  <li>cat nip</li>
  <li>laser pointers</li>
  <li>lasagna</li>
</ul>
<p>Top 3 things cats hate:</p>
<ol>
  <li>flea treatment</li>
  <li>thunder</li>
  <li>other cats</li>
</ol>
<form action="/submit-cat-photo">
  <input type="text" placeholder="cat photo URL" required>
  <button type="submit">Submit</button>
</form>