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Jun 24, 2016 am 11:43 AM

Horizontal center setting - fixed-width block element

When the element being set is a block element, using text-align: center will not work. , there are two situations at this time: fixed width block element and variable width block element. In this section, we first talk about fixed-width block elements.

Elements that meet the two conditions of fixed width and block shape can be centered by setting the "left and right margin" value to "auto". Let's look at an example of setting the div block element to be horizontally centered:

html code:

<body>  <div>我是定宽块状元素，哈哈，我要水平居中显示。</div></body>

css code:

<style>div{    border:1px solid red;/*为了显示居中效果明显为 div 设置了边框*/        width:500px;/*定宽*/    margin:20px auto;/* margin-left 与 margin-right 设置为 auto */}</style>

can also be written as:

margin-left:auto;margin-right:auto;

Note: The "upper and lower margin" of the element can be set at will of.

Summary of horizontal centering - variable width block element method (1)

In actual work, we will encounter the need to set "variable width block elements" Centering, such as paging navigation on a web page, because the number of paging is uncertain, we cannot limit its flexibility by setting the width.

There are three ways to center block elements with variable widths (these three methods are currently used more often):

Add table tag
Set display; inline method
Set position:relative and left:50%;

In this section we will talk about the first method:

Step 1: Add a table tag (including

, , ) outside the centered element that needs to be set.

Step 2: Set "left and right margins centered" for this table (this is the same method as the fixed-width block element).

For example:

html code:

<div><table>  <tbody>    <tr><td>    <ul>        <li><a href="#">1</a></li>        <li><a href="#">2</a></li>        <li><a href="#">3</a></li>    </ul>    </td></tr>  </tbody></table></div>

css code:

<style>table{    margin:0 auto;}ul{list-style:none;margin:0;padding:0;}li{float:left;display:inline;margin-right:8px;}</style>

Horizontal centering summary - variable width block element method (2)

Change the display of the block-level element to inline type, and then use text-align:center to achieve the centering effect. The following example:

html code:

<body><div class="container">    <ul>        <li><a href="#">1</a></li>        <li><a href="#">2</a></li>        <li><a href="#">3</a></li>    </ul></div></body>

css code:

<style>.container{    text-align:center;}.container ul{    list-style:none;    margin:0;    padding:0;    display:inline;}.container li{    margin-right:8px;    display:inline;}</style>

Compared with this method The advantage of the first method is that it does not need to add unsemantic tags and simplifies the nesting depth of tags, but there are also some problems: it changes the display type of block elements to inline, turning them into inline elements, so there are less Functions, such as setting the length value.

Summary of horizontal centering - variable width block element method (3)

By setting float to the parent element, and then setting position:relative and left:50 to the parent element %, the child element sets position:relative and left:-50% to achieve horizontal centering.

The code is as follows:

<body><div class="container">    <ul>        <li><a href="#">1</a></li>        <li><a href="#">2</a></li>        <li><a href="#">3</a></li>    </ul></div></body>

css code:

<style>.container{    float:left;    position:relative;    left:50%}.container ul{    list-style:none;    margin:0;    padding:0;        position:relative;    left:-50%;}.container li{float:left;display:inline;margin-right:8px;}</style>

This This method can retain block elements and still display them in the form of display:block. The advantage is that it does not add silent discussion tags and does not increase the nesting depth. However, its disadvantage is that position:relative is set, which brings certain side effects.

Vertical centering - a single line of text whose height is determined by the parent element

The method of vertical centering of a single line of text by which the height of the parent element is determined is by setting This is achieved by making the height and line-height of the parent element consistent. The following code:

<div class="container">    hi,imooc!</div>

css code:

<style>.container{    height:100px;    line-height:100px;    background:#999;}</style>

Vertically centered - multi-line text with a determined height of the parent element (Method 1)

Parent There are two ways to vertically center multi-line text, pictures, and block elements with a certain element height:

Method 1: Use insert table (including tbody, tr, td) tags, and set vertical-align at the same time :middle.

Speaking of vertical centering, there is a property vertical-align in CSS for vertical centering, but this style will only take effect when the parent element is td or th. So we have to insert the table tag again. Let’s take a look at an example:

html code:

<body><table><tbody><tr><td class="wrap"><div>    <p>看我是否可以居中。</p>    <p>看我是否可以居中。</p>    <p>看我是否可以居中。</p>    <p>看我是否可以居中。</p>    <p>看我是否可以居中。</p></div></td></tr></tbody></table></body>

css code:

table td{height:500px;background:#ccc}

因为 td 标签默认情况下就默认设置了 vertical-align 为 middle，所以我们不需要显式地设置了。

垂直居中-父元素高度确定的多行文本（方法二）

在 chrome、firefox 及 IE8 以上的浏览器下可以设置块级元素的 display 为 table-cell，激活 vertical-align 属性，但注意 IE6、7 并不支持这个样式。

html代码：

<div class="container">    <div>        <p>看我是否可以居中。</p>        <p>看我是否可以居中。</p>        <p>看我是否可以居中。</p>        <p>看我是否可以居中。</p>        <p>看我是否可以居中。</p>    </div></div>

css代码：

<style>.container{    height:300px;    background:#ccc;    display:table-cell;/*IE8以上及Chrome、Firefox*/    vertical-align:middle;/*IE8以上及Chrome、Firefox*/}</style>

这种方法的好处是不用添加多余的无意义的标签，但缺点也很明显，它的兼容性不是很好，不兼容 IE6、7。

隐性改变display类型

有一个有趣的现象就是当为元素（不论之前是什么类型元素，display:none 除外）设置以下 2 个句之一：

position : absolute
float : left 或 float:right

元素会自动变为以 display:inline-block 的方式显示，当然就可以设置元素的 width 和 height 了且默认宽度不占满父元素。

如下面的代码，小伙伴们都知道 a 标签是行内元素，所以设置它的 width 是没有效果的，但是设置为 position:absolute 以后，就可以了。

<div class="container">    <a href="#" title="">进入课程请单击这里</a></div>

css代码

<style>.container a{    position:absolute;    width:200px;    background:#ccc;}</style>

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