如何为下拉菜单文本和中心下拉菜单文本重复导航栏文本的下划线 CSS？

Question

问题 1

当我将鼠标悬停在下拉菜单中的导航栏文本上时，框底部会出现一条下划线。但是，我希望导航栏下拉菜单文本的过程发生。当用户将鼠标悬停在下拉菜单文本上而不是下划线位于文本底部时，它位于下拉菜单框的底部。我怎样才能做到这一点？

问题 2

如何使网页的导航栏下拉菜单文本在中心一个接一个地显示？

这是我的codepen，包含所有的 html 和 CSS。

以下 HTML

<nav class="navbar navbar-default">
    <div class="container-fluid">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="#"><img src="http://placehold.it/60x60" alt="Your Brand Name"></a>
        </div>

        <!-- Collect the nav links, forms, and other content for toggling -->
        <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
            <ul class="nav navbar-nav">
                    <li><a href="#">Home</a>
                    </li>
                    <li class="dropdown">
                        <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="true">About Me <span class="caret"></span></a>
                        <ul class="dropdown-menu">
                            <li><a href="#">Action</a>
                            </li>
                            <li><a href="#">Another action</a>
                            </li>
                            <li><a href="#">Something else here</a>
                            </li>
                            <li role="separator" class="divider"></li>
                            <li><a href="#">Separated link</a>
                            </li>
                            <li role="separator" class="divider"></li>
                            <li><a href="#">One more separated link</a>
                            </li>
                        </ul>
                    </li>
                    <li class="dropdown">
                        <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Units <span class="caret"></span></a>
                        <ul class="dropdown-menu">
                            <li><a href="#">Action</a>
                            </li>
                            <li><a href="#">Another action</a>
                            </li>
                            <li><a href="#">Something else here</a>
                            </li>
                            <li role="separator" class="divider"></li>
                            <li><a href="#">Separated link</a>
                            </li>
                            <li role="separator" class="divider"></li>
                            <li><a href="#">One more separated link</a>
                            </li>
                        </ul>
                    </li>
                    <li><a href="#">Clients</a>
                    </li>
                    <li><a href="#contact-me">Contact Me</a>
                    </li>
                </ul>
              </div><!-- /.navbar-collapse -->
            </div><!-- /.container-fluid -->
</nav>

Answer 1

将display: block; position: relative; 添加到li 和a 的下拉列表中。该边框基于绝对定位的元素，并且绝对定位的元素相对于最近的非静态定位的祖先进行定位。并且让它们都阻塞将确保它们在自己的线路上，并且您向父级应用的 text-align: center 将起作用。

body {
  margin: 0;
  padding: 0;
}

.navbar.navbar-default {
  background-color: #4D5061;
  height: 10vh;
}

.navbar.navbar-default ul {
  list-style-type: none;
  text-align: center;
}

.navbar.navbar-default ul li {
  display: inline-block;
}

.dropdown .dropdown-menu {
  background-color: #4D5061;
}

.dropdown .dropdown-menu a {
  color: white;
}

.navbar.navbar-default ul li a {
  display: inline-block;
  padding: 3.5vh 8px 4px;
  color: white;
  text-decoration: none;
  font-size: 14pt;
  font-family: 'Roboto', sans-serif;
}

.navbar.navbar-default ul li:after {
  content: '';
  position: absolute;
  right: 50%;
  bottom: 0;
  left: 50%;
  height: 3px;
  background-color: red;
  border-radius: 9px;
  transition: all .2s;
}

.navbar.navbar-default ul li a:hover {
  color: red;
}

.nav.navbar-nav,
.nav.navbar-nav>li {
  float: none;
}

.navbar.navbar-default ul li:hover:after {
  right: 0;
  left: 0;
}

a:hover {
  color: red;
  text-decoration: none;
}

#logo {
  padding-top: 2vh;
  padding-left: 20px;
  float: left;
}

section {
  position: relative;
}

.section1 {
  height: 90vh;
  background-color: #FFFFFF;
  text-align: center;
}

.section2 {
  height: 95vh;
  background-color: #A59E8C;
  text-align: center;
  color: white;
}

.contact_section {
  height: 93vh;
  background-color: grey;
}

.fa-angle-down {
  color: #4D5061;
  position: absolute;
  bottom: 0px;
}

footer {
  height: 5vh;
}

.navbar.navbar-default ul.dropdown-menu li,
.navbar.navbar-default ul.dropdown-menu li a {
  position: relative;
  display: block;
}

<!DOCTYPE html>
<html lang="en">

<html>

<head>
  <title>Liam Docherty | London Web Developer &amp; GFX designer</title>
  <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
  <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
</head>

<body>
  <nav class="navbar navbar-default">
    <div class="container-fluid">
      <!-- Brand and toggle get grouped for better mobile display -->
      <div class="navbar-header">
        <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">
                            <span class="sr-only">Toggle navigation</span>
                            <span class="icon-bar"></span>
                            <span class="icon-bar"></span>
                            <span class="icon-bar"></span>
                        </button>
        <a class="navbar-brand" href="#"><img src="http://placehold.it/60x60" alt="Your Brand Name"></a>
      </div>

      <!-- Collect the nav links, forms, and other content for toggling -->
      <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
        <ul class="nav navbar-nav">
          <li><a href="#">Home</a>
          </li>
          <li class="dropdown">
            <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="true">About Me <span class="caret"></span></a>
            <ul class="dropdown-menu">
              <li><a href="#">Action</a>
              </li>
              <li><a href="#">Another action</a>
              </li>
              <li><a href="#">Something else here</a>
              </li>
              <li role="separator" class="divider"></li>
              <li><a href="#">Separated link</a>
              </li>
              <li role="separator" class="divider"></li>
              <li><a href="#">One more separated link</a>
              </li>
            </ul>
          </li>
          <li class="dropdown">
            <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Units <span class="caret"></span></a>
            <ul class="dropdown-menu">
              <li><a href="#">Action</a>
              </li>
              <li><a href="#">Another action</a>
              </li>
              <li><a href="#">Something else here</a>
              </li>
              <li role="separator" class="divider"></li>
              <li><a href="#">Separated link</a>
              </li>
              <li role="separator" class="divider"></li>
              <li><a href="#">One more separated link</a>
              </li>
            </ul>
          </li>
          <li><a href="#">Clients</a>
          </li>
          <li><a href="#contact-me">Contact Me</a>
          </li>
        </ul>
      </div>
      <!-- /.navbar-collapse -->
    </div>
    <!-- /.container-fluid -->
  </nav>


  <section class="section1">
    <div class="hero"></div>


    <a href="#section2"><i class="fa fa-angle-down" style="font-size:100px;"></i></a>

  </section>
  <section class="section2" id="section2">


    <a href="#contact-me"> <i class="fa fa-angle-down" style="font-size:100px;"></i></a>

  </section>
  <section id="contact-me" class="contact_section">

  </section>
  <script src="https://code.jquery.com/jquery-3.1.1.js" integrity="sha256-16cdPddA6VdVInumRGo6IbivbERE8p7CQR3HzTBuELA=" crossorigin="anonymous"></script>
  <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>

</body>

<footer></footer>

</html>