display: inline/inline-block
将父元素（容器）设定 text-align: center； 即可左右置中。
display: block
将元素本身的 margin-left 和 margin-right 设定为auto; 即可左右置中。
方法一：Position: Absolute 对齐元素本身
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>上下左右居中对齐</title>
<style type="text/css">
#box {
width: 100px;
height: 100px;
border: 2px solid #0000FF;
}
#box {
position: absolute;
top: 50%;
left: 50%;
transform: translateX(-50%) translateY(-50%);
/* 或者这样写 */
right: 50%;
bottom: 50%;
transform: translateX(50%) translateY(50%);
}
</style>
</head>
<body>
<div id="box"></div>
</body>
</html>
方法二：Flexbox 对齐元素内容
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>上下左右居中对齐</title>
<style type="text/css">
#box {
width: 100px;
height: 100px;
border: 2px solid #0000FF;
}
body {
min-height: 100vh;
display: flex;
justify-content: center;
align-items: center;
}
</style>
</head>
<body>
<div id="box"></div>
</body>
</html>
方法三：Display : Table 对齐元素内容
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>上下左右居中对齐</title>
<style type="text/css">
#box {
width: 100px;
height: 100px;
border: 2px solid #0000FF;
display: inline-block;
}
body {
display: table;
width: 100%;
min-height: 100vh;
margin: 0;
}
.cell {
display: table-cell;
vertical-align: middle;
text-align: center;
}
</style>
</head>
<body>
<div class="cell">
<div id="box"></div>
</div>
</body>
</html>
