效果
思路
定义三个元素，文本框，下划线，占位文字。input 聚焦时通过 ~ 选中兄弟元素，利用 required 属性 + css 中的 valid 验证，判断 input 中是否有输入。写入过渡效果。
实现代码
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta
name="viewport"
content="initial-scale=1.0, user-scalable=no, width=device-width"
/>
<title>document</title>
<style>
.form-item {
width: 200px;
height: 30px;
margin: 200px auto;
position: relative;
border-bottom: 2px solid #000;
}
.form-item input:focus,
.form-item input:focus-visible,
.form-item input {
position: absolute;
top: 0;
left: 0;
border: none;
inset: 0;
outline: none;
}
.form-item label {
left: 0;
top: 0;
position: absolute;
transition: 0.4s ease;
}
.form-item .bar {
position: absolute;
display: block;
transform: translate(-50%, -50%);
background: #5264ae;
transition: 0.4s ease;
bottom: -4px;
left: 50%;
width: 0%;
height: 2px;
}
.form-item input:focus ~ .bar {
width: 100%;
}
.form-item input:valid ~ label,
.form-item input:focus ~ label {
color: #5264ae;
transform: translateY(-30px);
font-size: 16px;
}
</style>
</head>
<body>
<div class="form-item">
<input required id="username" type="text" />
<span class="bar"></span>
<label for="username">User Name</label>
</div>
<script></script>
</body>
</html>
