<!DOCTYPE HTML>
<html>
<head>
<meta charset=UTF-8 />
<title>Nothing</title>
<style type="text/css">
#post{
position: relative;
}
span{
position: absolute;
left: 0.625rem;
top: 0;
pointer-events: none;
transition: all 0.4s;
}
input:focus + span{
color: palevioletred;
transform: scale(0.8);
background-color: #fff;
padding: 4px;
top: -10px;
}
</style>
</head>
<body>
<div id="post">
<input type="text" id="txt" />
<span>郵箱</span>
</div>
</body>
</html>