:required

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨2015年7月⁩.

:required CSS 伪类表示任何设置了 required 属性的 <input>、<select> 或 <textarea> 元素。

尝试一下

label {
  display: block;
  margin-top: 1em;
}

.req {
  color: red;
}

*:required {
  background-color: gold;
}

<form>
  <label for="name">Name: <span class="req">*</span></label>
  <input id="name" name="name" type="text" required />

  <label for="birth">Date of Birth:</label>
  <input id="birth" name="birth" type="date" />

  <label for="origin"
    >How did you find out about us? <span class="req">*</span></label
  >
  <select id="origin" name="origin" required>
    <option>Google</option>
    <option>Facebook</option>
    <option>Advertisement</option>
  </select>
  <p><span class="req">*</span> - Required field</p>
</form>

这个伪类很有用，可以突出显示在提交表单之前必须具有有效数据的字段。

备注： :optional 伪类选择可选表单字段。

语法

css

:required {
  /* ... */
}

示例

必填字段具有红色边框

HTML

html

<form>
  <div class="field">
    <label for="url_input">Enter a URL:</label>
    <input type="url" id="url_input" />
  </div>

  <div class="field">
    <label for="email_input">Enter an email address:</label>
    <input type="email" id="email_input" required />
  </div>
</form>

CSS

css

label {
  display: block;
  margin: 1px;
  padding: 1px;
}

.field {
  margin: 1px;
  padding: 1px;
}

input:required {
  border-color: #800000;
  border-width: 3px;
}

input:required:invalid {
  border-color: #c00000;
}