背景
话不多说，我们能否用纯css实现以下效果:
答案是肯定的。
借助css:placeholder-shown :valid :invalid伪类及html5 input pattern 属性就可以实现，:placeholder-shown伪类目前兼容性如下：
直接上代码！
html:
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title>JS Bin</title>
</head>
<body>
<div class="input-fill-box">
<input class="input-fill" placeholder="邮箱" pattern="^[A-Za-z0-9\u4e00-\u9fa5]+@[a-zA-Z0-9_-]+(\.[a-zA-Z0-9_-]+)+$" required>
<a href="javascript:" class="clear">close</a>
<label class="input-label">邮箱</label>
</div>
</body>
</html>
专门建立的学习Q-q-u-n ⑦⑧④-⑦⑧③-零①② 分享学习方法和需要注意的小细节，互相交流学习，不停更新最新的教程和学习技巧（从零基础开始到WEB前端项目实战教程，学习工具，全栈开发学习路线以及规划）css:
.input-fill{
width: 100%;
margin: 0;
font-size: 16px;
line-height: 1.5;
outline: none;
padding: 20px 16px 6px;
border: 1px solid transparent;
background: #f5f5fa;
border-radius:10px;
transition: border-color .25s;
}
.input-fill:placeholder-shown::placeholder {
color: transparent;
}
.input-fill-box {
width: 50%;
position: relative;
}
.input-label {
position: absolute;
left: 16px; top: 14px;
pointer-events: none;
color:#BEC1D9;
padding: 0 2px;
transform-origin: 0 0;
pointer-events: none;
transition: all .25s;
}
.input-fill:not(:placeholder-shown) ~ .input-label,
.input-fill:focus ~ .input-label {
transform: scale(0.75) translate(0px, -14px);
}
.input-fill:focus{
border: 2px solid #1d31aa;
}
.clear{
position:absolute;
top:10px;
right:-20px;
display: none;
transition: all .25s;
}
.input-fill::-ms-clear { display: none; }
.input-fill:not(:placeholder-shown) + .clear { display: inline; }
.input-fill:valid {
border-color: green;
box-shadow: inset 5px 0 0 green;
}
.input-fill:not(:placeholder-shown):invalid {
border-color: red;
box-shadow: inset 5px 0 0 red;
}