一.水準居中(margin:0 auto;)
注意：被包裹的元素不能有浮動屬性，否則會失效。
示例代碼：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style>
body{margin: 0;}
.box{
width: 400px;
height: 400px;
border:1px solid red;
}
.item{
margin:0 auto;
width: 100px;
height: 100px;
background: green;
}
</style>
</head>
<body>
<div class="box">
<div class="item"></div>
</div>
</body>
</html>
效果圖：
二.水準居中(text-align:center;)
注意：沒有浮動的情況下，設定display:inline-block
示例代碼：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style>
body{margin: 0;}
.box{
width: 400px;
height: 400px;
border:1px solid red;
text-align:center;
}
.item{
display:inline-block;
width: 100px;
height: 100px;
background: green;
}
</style>
</head>
<body>
<div class="box">
<div class="item"></div>
</div>
</body>
</html>
三.水準垂直居中(方式1)
注意：子元素相對于父元素絕對定位，并且margin值減去自己寬高的一半(必須要知道子元素本身的寬高)
示例代碼：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style>
body{margin: 0;}
.box{
width: 400px;
height: 400px;
border:1px solid red;
position: relative;
}
.item{
position: absolute;
top: 50%;
left: 50%;
margin-top: -50px;
margin-left: -50px;
width: 100px;
height: 100px;
background: green;
}
</style>
</head>
<body>
<div class="box">
<div class="item"></div>
</div>
</body>
</html>
效果圖：
四.水準垂直居中(方式2)
注意：子元素相對于父元素絕對定位，并且margin值位auto(不受元素寬高所限制，比較好用)
示例代碼：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style>
body{margin: 0;}
.box{
width: 400px;
height: 400px;
border:1px solid red;
position: relative;
}
.item{
position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
top:0;
margin: auto;
width: 100px;
height: 100px;
background: green;
}
</style>
</head>
<body>
<div class="box">
<div class="item"></div>
</div>
</body>
</html>
五.水準垂直居中(方式3)
注意：将元素轉換成表格樣式，再利用表格的樣式來進行居中，設定diplay：table-cell(推薦使用)
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style>
body{margin: 0;}
.box{
width: 400px;
height: 400px;
border:1px solid red;
display: table-cell;
vertical-align: middle;
}
.item{
margin:0 auto;
width: 100px;
height: 100px;
background: green;
}
</style>
</head>
<body>
<div class="box">
<div class="item"></div>
</div>
</body>
</html>
六.水準垂直居中(方式4)
注意： 用絕對定位和transfrom，css3變形，有相容性問題
示例代碼：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style>
body{margin: 0;}
.box{
width: 400px;
height: 400px;
border:1px solid red;
position:relative;
}
.item{
position: absolute;
left: 50%;
top: 50%;
transform: translate(-50%,-50%);
width: 100px;
height: 100px;
background: green;
}
</style>
</head>
<body>
<div class="box">
<div class="item"></div>
</div>
</body>
</html>
七.水準垂直居中(方式5)
注意：css3中的flex屬性，有相容性問題
示例代碼：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style>
body{margin: 0;}
.box{
width: 400px;
height: 400px;
border:1px solid red;
display: flex;
justify-content: center;
align-items: center;
}
.item{
width: 100px;
height: 100px;
background: green;
}
</style>
</head>
<body>
<div class="box">
<div class="item"></div>
</div>
</body>
</html>