1. Text Align
对象：行内元素， div内文字，匿名容器（没有被标签包裹的文本节点）
效果：水平居中
方法：对其父元素引用text-align
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
body{
margin: 0;
padding: 0;
background: #eaeaea;
}
.container{
width: 1000px;
height: 800px;
background: white;
margin: 0 auto;
text-align: center;
}
.box{
width:300px;
height:160px;
background: yellowgreen;
}
</style>
</head>
<body>
<div class="container">
test
<div class="box">test</div>
</div>
</body>
</html>
2. Margin Auto：
margin: 0 auto;
等价于margin-left:auto; margin-right:auto;
上述方法是左右对齐，对于上下对齐，需要计算上下margin，并且设置父元素为overflow:hidden
overflow：详解 overflow：解决外边距坍塌
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
body {
margin: 0;
padding: 0;
background: #eaeaea;
}
.container {
width: 1000px;
height: 600px;
background: white;
overflow: hidden;
}
.box {
width: 200px;
height: 200px;
background:deepskyblue;
margin:200px auto;
}
</style>
</head>
<body>
<div class="container">
<div class="box" ></div>
</div>
</body>
</html>
其他方法是
position: absolute;
top: 0;
bottom: 0;
left:0;
right:0;
margin: auto;
3. Display Flex：justify-content 主轴对齐方式 align-items:侧轴对齐方式
不涉及文档流。
<!DOCTYPE html>
<main lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
.box1{
background: yellowgreen;
width: 1000px;
height: 600px;
display: flex;
justify-content: center;
align-items: center;
}
</style>
</head>
<body>
<div class="box1">
<div class="box2">form</div>
</div>
</body>
</main>
4. Position Absolute
position:relative用于父元素， position:absolute用于子元素， 子元素会依照父层的边界进行定位的, 不然position:absolute 会逐层向上寻找设置了position:relative的元素边界, 直到body元素.
适合居中的元素高度不会超过视口，否则顶部会被切掉。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
.box1 {
height: 200px;
width: 200px;
background: deepskyblue;
position: relative;
}
.box2 {
height: 100px;
width: 100px;
background: yellowgreen;
position: absolute;
top:50%;
left:50%;
transform: translate(-50%, -50%);
}
</style>
</head>
<body>
<div class="box1">
<div class="box2"></div>
</div>
</body>
</html>