按钮或菜单项添加 hover 时下划线动画，可以给界面增色不少。
一种巧妙的方式是通过after伪元素来实现（在线示例）：
html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<meta http-equiv="X-UA-Compatible" content="ie=edge" />
<title>HTML + CSS</title>
<link rel="stylesheet" href="styles.css" />
<style>
.c-text-button {
position: relative;
display: inline-flex;
align-items: center;
padding: 2px 4px;
cursor: pointer;
font-size: 14px;
transition: all 0.3s ease;
}
.c-text-button::after {
position: absolute;
bottom: 0;
left: 0;
width: 0;
height: 1px;
background-color: currentcolor;
content: "";
transition: width 0.3s ease;
}
.c-text-button:hover {
opacity: 1;
transform: translateY(-1px);
}
.c-text-button:hover::after {
width: 100%;
}
</style>
</head>
<body>
<div class="c-text-button">button</div>
</body>
</html>