来源：http://www.ido321.com/824.html
一、水平居中
1、内联元素居中：相对父级块级元素居中对齐
1: .center-children {
2: text-align: center;
3: }
2、块级元素居中：设置margin-left和margin-right为auto让它居中（同时还要设置width，否则它就会承满整个容器，无法看出居中效果）
1: .center-me {
2: margin: 0 auto;
3: }
如果有很多块级元素需要水平居中成一行，最好使用一个不同的display类型。这是一个使用inline-block和flex的例子。
演示地址：http://jsfiddle.net/web_code/5fvrwwk1/embedded/result/
1:
2: 3: i'm an element that is block-like with my siblings and we're centered in a row.
4:
5: 6: i'm an element that is block-like with my siblings and we're centered in a row. i have more content in me than my siblings do.
7:
8: 9: i'm an element that is block-like with my siblings and we're centered in a row.
10:
11:
12:
13: 14: i'm an element that is block-like with my siblings and we're centered in a row.
15:
16: 17: i'm an element that is block-like with my siblings and we're centered in a row. i have more content in me than my siblings do.
18:
19: 20: i'm an element that is block-like with my siblings and we're centered in a row.
21:
22:
css:
1: body {
2: background: #f06d06;
3: font-size: 80%;
4: }
5: main {
6: background: white;
7: margin: 20px 0;
8: padding: 10px;
9: }
10: main div {
11: background: black;
12: color: white;
13: padding: 15px;
14: max-width: 125px;
15: margin: 5px;
16: }
17: .inline-block-center {
18: text-align: center;
19: }
20: .inline-block-center div {
21: display: inline-block;
22: text-align: left;
23: }
24: .flex-center {
25: display: flex;
26: justify-content: center;
27: }
二、垂直居中
1、内联元素：设置相等的上下padding，或者利用line-height
1: .link {
2: padding-top: 30px;
3: padding-bottom: 30px;
4: }
文本不会换行的情况下，可以使用line-height，让其与height相等去对齐文本。
1: .center-text-trick {
2: height: 100px;
3: line-height: 100px;
4: white-space: nowrap;
5: }
多行的文本也可以利用上下等padding的方式也可以让多行居中，但是如果这方法没用，你可以让这些文字的容器按table cell模式显示，然后设置文字的vertical-align属性对齐，
演示地址： http://jsfiddle.net/web_code/5fvrwwk1/1/embedded/result/
html:
1:
2: 3: 4: i'm vertically centered multiple lines of text in a real table cell.
5:
6:
7:
8: 9: i'm vertically centered multiple lines of text in a css-created table layout.
10:
css
1: body {
2: background: #f06d06;
3: font-size: 80%;
4: }
5: table {
6: background: white;
7: width: 240px;
8: border-collapse: separate;
9: margin: 20px;
10: height: 250px;
11: }
12: table td {
13: background: black;
14: color: white;
15: padding: 20px;
16: border: 10px solid white;
17: /* default is vertical-align: middle; */
18: }
19: .center-table {
20: display: table;
21: height: 250px;
22: background: white;
23: width: 240px;
24: margin: 20px;
25: }
26: .center-table p {
27: display: table-cell;
28: margin: 0;
29: background: black;
30: color: white;
31: padding: 20px;
32: border: 10px solid white;
33: vertical-align: middle;
34: }
2、块级元素
若元素有固定高度，可以这样垂直居中
1: .parent {
2: position: relative;
3: }
4: .child {
5: position: absolute;
6: top: 50%;
7: height: 100px;
8: margin-top: -50px; /* 如果没有使用: border-box; 的盒子模型则需要设置这个 */
9: }
如果不知道元素高度，则这样
1: .parent {
2: position: relative;
3: }
4: .child {
5: position: absolute;
6: top: 50%;
7: transform: translatey(-50%);
8: }
也可以使用flexbox
1:
2: 3: i'm a block-level element with an unknown height, centered vertically within my parent.
4:
5:
1: body {
2: background: #f06d06;
3: font-size: 80%;
4: }
5: main {
6: background: white;
7: height: 300px;
8: width: 200px;
9: padding: 20px;
10: margin: 20px;
11: display: flex;
12: flex-direction: column;
13: justify-content: center;
14: resize: vertical;
15: overflow: auto;
16: }
17: main div {
18: background: black;
19: color: white;
20: padding: 20px;
21: resize: vertical;
22: overflow: auto;
23: }
三、同时水平和垂直居中
1、元素有固定高度和宽度：先绝对居中，然后上移和左移50%的宽度即可
1: //这种方案有极好的跨浏览器支持。
2: .parent {
3: position: relative;
4: }
5: .child {
6: width: 300px;
7: height: 100px;
8: padding: 20px;
9: position: absolute;
10: top: 50%;
11: left: 50%;
12: margin: -70px 0 0 -170px;
13: }
2、元素的高度和宽度未知或可变的：使用transofrm属性在两个方向都平移负50%
1: .parent {
2: position: relative;
3: }
4: .child {
5: position: absolute;
6: top: 50%;
7: left: 50%;
8: transform: translate(-50%, -50%);
9: }