如何使用 CSS 为导航栏设置下划线格式

Question

我想出了当我将鼠标悬停在单词上时如何在导航栏的一部分下方添加下划线的方法。当我在该页面上时，我试图以相同的格式制作它，以便下划线保留在那里。我想不通。会喜欢一些帮助。谢谢:)

*{
    margin:0;
    padding:0;
    border:0;
}


.topnav {
    background-color: purple;
    overflow: hidden;
  }
  
  .topnav a {
    float: left;
    color: #f2f2f2;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
    font-size: 17px;
    position: relative;
  }

  .topnav a:before {
    content: "";
    position: absolute;
    width: 84%;
    height: 2px;
    bottom: 3px;
    left: 8%;
    background-color: white;
    visibility: hidden;
    transform: scaleX(0);
    transition: all 0.3s ease-in-out 0s;
  }

 .topnav a:hover:before {
    visibility: visible;
    transform: scaleX(1);
  }

<!DOCTYPE html>
<html lang="en">
<link rel="stylesheet" href="style.css">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>
    Change
  </title>
</head>

<body>
  <div class="topnav">
    <a class="active" href="index.html">Home</a>
    <a href="#news">DontUse</a>
    <a href="#contact">DontUse</a>
    <a href="about.html">About</a>
  </div>
  <p>
    This is the home page
  </p>
</body>

</html>

Answer 1

给你，使用 jquery。还制作了一个新的活动菜单 class .topnav a.active-menu:before

$("a").click(function(){
   $("a.active-menu").removeClass("active-menu");
   $(this).addClass("active-menu");
});

*{
    margin:0;
    padding:0;
    border:0;
}


.topnav {
    background-color: purple;
    overflow: hidden;
  }
  
  .topnav a {
    float: left;
    color: #f2f2f2;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
    font-size: 17px;
    position: relative;
  }

  .topnav a:before {
    content: "";
    position: absolute;
    width: 84%;
    height: 2px;
    bottom: 3px;
    left: 8%;
    background-color: white;
    visibility: hidden;
    transform: scaleX(0);
    transition: all 0.3s ease-in-out 0s;
  }

 .topnav a:hover:before {
    visibility: visible;
    transform: scaleX(1);
  }
.topnav a.active-menu:before {
    content: "";
    position: absolute;
    width: 84%;
    height: 2px;
    bottom: 3px;
    left: 8%;
    background-color: white;
    visibility: visible;
    transform: scaleX(1);
    transition: all 0.3s ease-in-out 0s; 
    }

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<!DOCTYPE html>
<html lang="en">
<link rel="stylesheet" href="style.css">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>
    Change
  </title>
</head>

<body>
  <div class="topnav" id="topnav">
    <a class="active-menu" href="index.html">Home</a>
    <a class="link" href="#news">DontUse</a>
    <a class="link" href="#contact">DontUse</a>
    <a class="link" href="about.html">About</a>
  </div>
  <p>
    This is the home page
  </p>
</body>

</html>

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