1. 口诀：子绝父相定位
!DOCTYPE html>
<body>
<div>
<input value="对齐了吗">
</div>
</body>
<style>
div{
position: relative;
height: 200px;
background-color: grey;
}
input{
position:absolute;
top:50%;
left:50%;
transform: translate(-50%,-50%);
}
</style>
</html>
结果
2.利用display：flex
<!DOCTYPE html>
<body>
<div>
<input value="对齐了吗">
</div>
</body>
<style>
div{
height: 200px;
background-color: grey;
display: flex;
justify-content:center;
align-items:Center;
}
</style>
</html>