css - 尝试制作响应式菜单，但在::before 和::after 方面遇到问题

Question

我有这支笔:https://codepen.io/cesarnascimento/pen/qvLmPG

他有一个红色框，当您单击时，它应该会显示菜单。它工作得很好，我尝试使用 ::before 和 ::after 为菜单制作一行，但它不起作用。

这是我的代码:

header {
  position: fixed;
  width: 100%;
  z-index: 999;
}
header .logo {
  margin: 0;
}
header .nav-toggle {
  display: none;
}
header .nav-toggle:checked ~ nav {
  display: block;
}
header nav {
  position: absolute;
  width: 100%;
  text-align: left;
  top: 100%;
  left: 0;
  display: none;
}
header nav ul {
  margin: 0;
  padding: 0;
  list-style: none;
}
header nav ul li {
  margin-bottom: 1em;
  margin-left: 1em;
}
header nav ul li a {
  color: cyan;
  text-decoration: none;
  font-size: 1.2rem;
  text-transform: uppercase;
}
header nav ul li a:hover {
  color: #000;
}
header .nav-toggle-label {
  position: absolute;
  top: 0;
  left: 0;
  margin-left: 1em;
  border: 1px solid red;
  height: 100%;
  display: flex;
  align-items: center;
}
header .nav-toggle-label span {
  display: block;
  width: 2em;
  height: 2px;
  border-radius: 2px;
  position: relative;
}
header .nav-toggle-label span::before {
  content: "";
  display: block;
  width: 2em;
  height: 2px;
  border-radius: 2px;
  position: absolute;
}
header .nav-toggle-label span::after {
  content: "";
  display: block;
  width: 2em;
  height: 2px;
  border-radius: 2px;
  position: absolute;
}

<header>
  <h1 class="logo">Logoe</h1>
  <input type="checkbox" id="nav-toggle" class="nav-toggle" />
  <nav>
    <ul>
      <li>
        <a href="#">Monitoria</a>
      </li>
      <li>
        <a href="#">Fale comigo</a>
      </li>
    </ul>
  </nav>
  <label for="nav-toggle" class="nav-toggle-label">
    <span></span>
  </label>
</header>

使用 Chrome Developer，我可以看到该行，但它不会出现在我的前面。我在这里缺少什么？

Answer 1

您需要为 ::before 和 ::after 元素设置一个 background-color。如果你不添加 background-color 背景是透明的，元素是不可见的:

header {
  position: fixed;
  width: 100%;
  z-index: 999;
}
header .logo {
  margin: 0;
}
header .nav-toggle {
  display: none;
}
header .nav-toggle:checked ~ nav {
  display: block;
}
header nav {
  position: absolute;
  width: 100%;
  text-align: left;
  top: 100%;
  left: 0;
  display: none;
}
header nav ul {
  margin: 0;
  padding: 0;
  list-style: none;
}
header nav ul li {
  margin-bottom: 1em;
  margin-left: 1em;
}
header nav ul li a {
  color: cyan;
  text-decoration: none;
  font-size: 1.2rem;
  text-transform: uppercase;
}
header nav ul li a:hover {
  color: #000;
}
header .nav-toggle-label {
  position: absolute;
  top: 0;
  left: 0;
  margin-left: 1em;
  border: 1px solid red;
  height: 100%;
  display: flex;
  align-items: center;
}
header .nav-toggle-label span {
  display: block;
  width: 2em;
  height: 2px;
  border-radius: 2px;
  position: relative;
  background:#000;
}
header .nav-toggle-label span::before {
  content: "";
  background:#000;
  display: block;
  width: 2em;
  height: 2px;
  border-radius: 2px;
  position: absolute;
  top:-6px;
}
header .nav-toggle-label span::after {
  content: "";
  background:#000;
  display: block;
  width: 2em;
  height: 2px;
  border-radius: 2px;
  position: absolute;
  top:6px;
}

<header>
  <h1 class="logo">Logoe</h1>
  <input type="checkbox" id="nav-toggle" class="nav-toggle" />
  <nav>
    <ul>
      <li>
        <a href="#">Monitoria</a>
      </li>
      <li>
        <a href="#">Fale comigo</a>
      </li>
    </ul>
  </nav>
  <label for="nav-toggle" class="nav-toggle-label">
    <span></span>
  </label>
</header>