用 CSS 将一个容器居中

1. position + margin
2. display:table-cell
3. position+transform
4. flex;align-items:center;justtify-contennt:center
5. display:flex;margin:auto
6. 纯 position
7. 兼容低版本浏览器，不固定宽高

1.position + margin

<!-- html -->
<div class="wrap">
  <div class="center"></div>
</div>

/**css**/
.wrap {
  width: 200px;
  height: 200px;
  background: yellow;
  position: relative;
}
.wrap .center {
  width: 100px;
  height: 100px;
  background: green;
  margin: auto;
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
}

兼容性：主流浏览器均支持，IE6 不支持

2.display:table-cell

<!-- html -->
<div class="wrap">
  <div class="center"></div>
</div>

/*css*/
.wrap {
  width: 200px;
  height: 200px;
  background: yellow;
  display: table-cell;
  vertical-align: middle;
  text-align: center;
}

.center {
  display: inline-block;
  vertical-align: middle;
  width: 100px;
  height: 100px;
  background: green;
}

3.position + transform

<!-- html -->
<div class="wrap">
  <div class="center"></div>
</div>

/* css */
.wrap {
  position: relative;
  background: yellow;
  width: 200px;
  height: 200px;
}

.center {
  position: absolute;
  background: green;
  top: 50%;
  left: 50%;
  -webkit-transform: translate(-50%, -50%);
  transform: translate(-50%, -50%);
  width: 100px;
  height: 100px;
}

兼容性：ie9 以下不支持 transform，手机端表现的比较好。

4.flex;align-items: center;justify-content: center

<!-- html -->
<div class="wrap">
  <div class="center"></div>
</div>

/* css */
.wrap {
    background: yellow;
    width: 200px;
    height: 200px;
    display: flex;
    align-items: center;
    justify-content: center;
}

.center {
    background: green;
    width: 100px;
    height: 100px;
}

移动端首选

5.display:flex;margin:auto

<!-- html -->
<div class="wrap">
    <div class="center"></div>
</div>

/* css */
.wrap {
    background: yellow;
    width: 200px;
    height: 200px;
    display: flex;
}

.center {
    background: green;
    width: 100px;
    height: 100px;
    margin: auto;
}

移动端首选

6.纯 position

<!-- html -->
<div class="wrap">
    <div class="center"></div>
</div>

/* css */
.wrap {
    background: yellow;
    width: 200px;
    height: 200px;
    position: relative;
}
/**方法一**/
.center {
    background: green;
    position: absolute;
    width: 100px;
    height: 100px;
    left: 50px;
    top: 50px;
}
/**方法二**/
.center {
    background: green;
    position: absolute;
    width: 100px;
    height: 100px;
    left: 50%;
    top: 50%;
  margin-left:-50px;
  margin-top:-50px;
}

兼容性：适用于所有浏览器

方法一计算公式如下：

子元素（center）的 left 值计算公式：left = (父元素的宽 - 子元素的宽 ) / 2=(200-100) / 2=50px

子元素（center）的 top 值计算公式：top = (父元素的高 - 子元素的高 ) / 2=(200-100) / 2=50px

方法二计算公式：

left 值固定为 50%

子元素的 margin-left= -（子元素的宽/2）=-100/2= -50px

top 值也一样，固定为 50%

子元素的 margin-top= -（子元素的高/2）=-100/2= -50px

7.兼容低版本浏览器，不固定宽高

<!-- html -->
<div class="table">
  <div class="tableCell">
    <div class="content">不固定宽高，自适应</div>
  </div>
</div>

/*css*/
.table {
  height: 200px; /*高度值不能少*/
  width: 200px; /*宽度值不能少*/
  display: table;
  position: relative;
  float: left;
  background: yellow;
}

.tableCell {
  display: table-cell;
  vertical-align: middle;
  text-align: center;
  *position: absolute;
  padding: 10px;
  *top: 50%;
  *left: 50%;
}
.content {
  *position: relative;
  *top: -50%;
  *left: -50%;
  background: green;
}

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