自己对知识的了解，留作面试用，如有错误请指正
水平居中：
行内元素：给父元素设置 text-align: center
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.p{
text-align: center;
}
.box{
width: 50px;
height: 50xp;
background-color: brown;
}
</style>
</head>
<body>
<div class="p">
<span class="box">一个盒子</span>
</div>
</body>
</html>
块级元素：已知width, margin:0 auto
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.box{
width: 50px;
margin: 0 auto;
background-color: brown;
}
</style>
</head>
<body>
<div class="box">一个盒子</div>
</body>
</html>
绝对定位：left+tranxlateX
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.box{
position: absolute;
left: 50%;
transform: translateX(-50%);
background-color: brown;
}
</style>
</head>
<body>
<div class="box">一个盒子</div>
</body>
</html>
flex布局：父元素justify-content: center
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.p{
display: flex;
justify-content: center;
}
.box{
background-color: brown;
}
</style>
</head>
<body>
<div class="p">
<div class="box">一个盒子</div>
</div>
</body>
</html>
# 垂直居中
单行文字：padding/line-height=height
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.box{
height: 200px;
line-height: 200px;
/* padding: 100px; */
background-color: brown;
}
</style>
</head>
<body>
<div class="box">文字垂直居中</div>
</body>
</html>
绝对定位：
子绝父相：
1、子元素上下为0，margin：auto 已知高度
2、子元素 top:50%,（margin-top:height一半/translateY(-50%)）未知高度
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.p{
position: relative;
background-color: aqua;
height: 200px;
}
.box{
position: absolute;
top:50%;
/* margin-top: -100px; */
transform: translateY(-50%);
/* top: 0;
bottom: 0;
margin: auto; */
height: 200px;
background-color: brown;
}
</style>
</head>
<body>
<div class="p">
<div class="box">一个盒子</div>
</div>
</body>
</html>
flex布局：
未知高度，父元素align-items: center;
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.p{
display: flex;
align-items: center;
background-color: aqua;
height: 500px;
}
.box{
/* height: 22px; */
background-color: brown;
}
</style>
</head>
<body>
<div class="p">
<div class="box">一个盒子</div>
</div>
</body>
</html>
上下左右居中
flex：父元素align-items: center;justify-content: center;
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.p{
display: flex;
align-items: center;
justify-content: center;
background-color: aqua;
height: 500px;
}
.box{
/* height: 22px; */
background-color: brown;
}
</style>
</head>
<body>
<div class="p">
<div class="box">一个盒子</div>
</div>
</body>
</html>
定位：子绝父相
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.p{
position: relative;
background-color: aqua;
height: 500px;
}
.box{
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%,-50%);
/* height: 22px; */
background-color: brown;
}
</style>
</head>
<body>
<div class="p">
<div class="box">一个盒子</div>
</div>
</body>
</html>
