<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        display: table-cell;    /* 轉(zhuǎn)變成表格 */
        text-align: center;     /* 水平 */
        vertical-align: middle; /* 垂直 */
    }
    #child {
        background-color: blue;
        color: white;

        display: inline;        /* 子元素設(shè)置為行內(nèi)或行內(nèi)塊 */
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child">內(nèi)容</div>
</div>

<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        position: relative;     /* 設(shè)置相對(duì)定位 */
    }
    #child {
        height: 50px;
        width: 50px;
        color: white;
        background-color: blue;

        /* 絕對(duì)定位，4 個(gè)方向設(shè)置為 0 后，margin 設(shè)為 auto */
        position: absolute;
        left: 0;
        top: 0;
        right: 0;
        bottom: 0;
        margin: auto;
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child"></div>
</div>

<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        display: flex;          /* 父元素轉(zhuǎn)換為彈性盒 */
        display: -webkit-flex;  /* Safari */
    }
    #child {
        height: 50px;
        width: 50px;
        background-color: blue;

        margin: auto;
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child"></div>
</div>

<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        position: relative;     /* 設(shè)置相對(duì)定位 */
    }
    #child {                      /* 子元素已知自身寬高的情況下 */
        background-color: blue;

        width: 50px;
        height: 50px;
        margin-top: -25px;
        margin-left: -25px;
        position: absolute;
        left: 50%;
        top: 50%;
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child"></div>
</div>

<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        position: relative;     /* 設(shè)置相對(duì)定位 */
    }
    #child {                      /* 子元素未知自己的寬高，使用 transform 的 translate */
        border: 1px solid blue;

        position: absolute;
        top: 50%;
        left: 50%;
        -webkit-transform: translate(-50%,-50%);
        -ms-transform: translate(-50%,-50%);
        -o-transform: translate(-50%,-50%);
        transform: translate(-50%,-50%);
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child">
        <div id="content">
            內(nèi)容1
            <br/>
            內(nèi)容2
        </div>
    </div>
</div>

<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        display: flex;          /* 父元素轉(zhuǎn)換為彈性盒 */
        display: -webkit-flex;  /* Safari */
        justify-content: center;/* 水平 */
        align-items: center;    /* 垂直 */
    }
    #child {
        height: 50px;
        width: 50px;
        background-color: blue;
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child"></div>
</div>