之前看到很多人一直都問這個問題，不過當時我沒當一回事，因爲在 CSS 中要垂直居中，多數是在有高度的情況下，或者容器高度不定的情況下才用，看上去比較舒服，而且實現的方法也不少，不一定要拘泥於和 table 佈局一樣。不過最近有人問了幾個例子，看來對此的需求還不少。現在就把我經驗拿出來分享一下，希望大家鼓鼓掌。
首先，要有一個概念：凡是 table 佈局可以實現的，CSS 一定可以實現。CSS 可以實現的，table 未必能做到。
現在來幾個例子：
一、單行內容的居中
只考慮單行是最簡單的，無論是否給容器固定高度，只要給容器設置 line-height 和 height，並使兩值相等，再加上 over-flow: hidden 就可以了
height: 4em;
line-height: 4em;
overflow: hidden;
}
優點：
1. 同時支持塊級和內聯極元素
2. 支持所有瀏覽器
缺點：
1. 只能顯示一行
2. IE中不支持<img>等的居中
要注意的是：
1. 使用相對高度定義你的 height 和 line-height
2. 不想毀了你的佈局的話，overflow: hidden 一定要
爲什麼？
請比較以下兩個例子：
<br/>
<br/>
<p style="background: #090; color: #00f; font: bold 12px/2em Helvertica,Arial,sans-serif; height:2em; width:370px; overflow: hidden;">Lorem ipsum dolor sit amet, consectetuer adipiscing elit.</p>
[Ctrl+A 全部選擇 提示：你可先修改部分代碼，再按運行]
上一個高度是用的絕對單位px，並且沒有隱藏溢出，下一個高度用的單位是相對單位em，並且隱藏了溢出。如果你的瀏覽器支持放大字體，那麼盡情地放大字體，看看會出現什麼效果。
二、多行內容居中，且容器高度可變
也很簡單，給出一致的 padding-bottom 和 padding-top 就行
padding-top: 24px;
padding-bottom: 24px;
}
優點：
1. 同時支持塊級和內聯極元素
2. 支持非文本內容
3. 支持所有瀏覽器
缺點：
容器不能固定高度
三、把容器當作表格單元
CSS 提供一系列diplay屬性值，包括 display: table, display: table-row, display: table-cell 等，能把元素當作表格單元來顯示。這是再加上 vertical-align: middle, 就和表格中的 valign="center" 一樣了。
display: table-cell;
height: 300px;
vertical-align: middle;
}
可惜IE不支持這些屬性，不過在其他瀏覽器上顯示效果非常完美。
要注意的是：和一個合法的<td>元素必須在<table>裏一樣，display: table-cell 元素必須作爲 display: table 的元素的子孫出現。
優點：
不用說了吧，就是表格，效果和表格一模一樣
缺點：
IE下無效
四、以毒攻毒！用 IE 的 bug 解決 IE 中的絕對居中
先不得不說一句，IE 真的是個很爛的瀏覽器，CSS1中的定義都不支持，害得要我們轉個大圈子來造居中。不過就像我說的，凡是 table 佈局可以實現的，CSS 一定可以實現，即使在 IE 裏也不例外。我研究 IE layout 模式多年，還是找出了一個可以在 IE 中絕對居中的方法。這個方法就是基於 IE layout 的 bug，也可以算以毒攻毒。至於原理，不要問我，這是獨門祕學，何況三言兩語也講不清楚，只要好用就行
height: 300px;
position: relative;
}
.middle-demo-4 div{
position: absolute;
top: 50%;
left: 0;
}
.middle-demo-4 div div{
position: relative;
top: -50%;
left: 0;
}
五、整合三和四，寫出支持所有瀏覽器的垂直居中容器！
思路是利用 IE 和 非IE 瀏覽器的 CSS hack, 整合三和四的CSS，寫出兼容主流瀏覽器的垂直居中容器。具體代碼就不給出了，大家權當作練習練習。例子可以在下面的附錄中找到。
最終實測支持的瀏覽器：IE6+, Mozilla 1.7, Netscape Navigator 8, Opera 8.0+, Firefox 1.0+ 和 Safari 1.0+IE5 下需要加上對合適模型的補正。
推測支持的瀏覽器：Mozilla 1.5+, Netscape Navigator 7+, Opera 7+
未測試瀏覽器：Konqueror
最後附上自己寫的，所有居中佈局的範例網頁，大家不明白可以參考。
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Demo:: vertical align: middle</title>
<meta lang="en" name="author" content="Spenser Lee" />
<meta lang="en" name="copyright" content="(c)2006 Liberty Studio" />
<style media="screen" type="text/css">
html,body,div,h1,h2,pre,dd,ol{margin: 0;padding: 0;border: 0}
html{min-width: 779px}
body{background: #fff;color: #596480;text-align: center}
div#main-wrapper{padding: 12px 5px;width: 769px;margin: 0 auto}
div,code,p,h1,h2,address,dt,dd,li{font: normal 12px/1.5em Tahoma,"Lucida Grande",Helvetica,Verdana,Lucida,Arial,"Arial Unicode",sans-serif,serif}
h1{font-size: 22px;font-weight: bold;border-left: 12px solid #324f96;background: #e0eaf4;color: #4868a9;height: 4em;line-height: 4em;padding: 0 12px;overflow: hidden; text-align: left}
h2{font-size: 12px;font-weight: bold;background: #c0014e;color: #fff;height: 2.5em;line-height: 2.5em;padding: 0 24px;overflow: hidden;margin: 12px 0;text-align: left}
h2 a{color: #fff;background: transparent}
h2 a:hover{text-decoration: none}
p{margin: 6px 0;padding: 0 12px 0 24px;text-indent: 2em;text-align: left}
p.snap-back{text-align: right}
code{display: block;font-family: "Courier New", Courier, monospace, mono, serif;margin: 12px auto;padding: 12px;border: 1px solid #596480;color: inherit;background: #f6f6f6;text-align: left;white-space: pre;width: 350px}
strong{font-weight: bold}
em{font-style: italic}
address{display: block;padding: 0 12px;margin: 12px 0;text-align: right}
dl{margin: 6px 0;padding: 0 12px 0 24px;text-align: left}
dt{margin: 0;text-indent: 2em;font-weight: bold}
dd{margin-left: 24px;text-indent: 2em}
li{list-style: square inside none;text-align: left}
ol#table-of-content{padding-left: 24px}
a{color: #c0014e;background: transparent;text-decoration: none}
a:hover{text-decoration: underline}
div.demo{width: 400px;margin: 12px auto;border: 1px solid #596480;color: inherit;background: #ffc}
div.demo p{text-align: left;margin: 24px;text-indent: 0}
p#demo-1{margin: 12px auto;padding: 0 12px;width: 400px;text-indent: 0;border: 1px solid #596480;color: inherit;background: #ffc;line-height: 4em;height: 4em;overflow: hidden}
div#demo-2{padding: 50px 0}
div#demo-4, div#demo-5{height: 300px;position: relative}
div#demo-4 div, div#demo-5 div{position: absolute;top: 50%;left: 0}
div#demo-4 p, div#demo-5 p{position: relative;top: -50%}
div#demo-3{display: table;height: 300px;border-collapse: collapse}
div#main-wrapper>div#demo-5{position: static;display:table}
div#main-wrapper>div#demo-5>div{display:table-cell;vertical-align:middle;position:static}
div#demo-3>div{display: table-row}
div#demo-3>div>div{display: table-cell;vertical-align: middle}
span.property{font-family: "Courier New", Courier, monospace, mono, serif;border-bottom: 1px dotted #596480;background: #ffc;color: #c0014e}
p.copyright{line-height: 3em;text-align: center;border-top: 1px dotted #596480}
</style>
</head>
<body>
<div id="main-wrapper">
<h1>Demo of middled vertical align</h1>
<address>
Author: Spenser Lee, Liberty Studio<br />
Originally posted on <a href="http://www.blueidea.com/bbs/" title="BlueIdea Forum">BlueIdea Forum</a>
</address>
<h2><a id="bullet-0">Table of centents:</a></h2>
<ol id="table-of-content">
<li><a href="#bullet-1" title="Single line countainer with/without a fixed height">Single line countainer with/without a fixed height</a></li>
<li><a href="#bullet-2" title="Align multi-line container which does not have a fixed height">Align multi-line container which does not have a fixed height</a></li>
<li><a href="#bullet-3" title="Simulating table layout in container with a fixed height">Simulating table layout in container with a fixed height</a></li>
<li><a href="#bullet-4" title="IE's solution">IE's solution</a></li>
<li><a href="#bullet-5" title="A perfect compounded sample">A perfect compounded sample</a></li>
</ol>
<h2><a id="bullet-1">Case One: Single line countainer with/without a fixed height</a></h2>
<p>If you only want to display a container which only holds a single line of text, you can set <span class="property">line-height</span> property to <span class="property">height</span> property, then set <span class="property">overflow</span> to hidden.</p>
<p><strong>Sample:</strong></p>
<p id="demo-1">Lorem ipsum dolor sit amet, consectetuer adipiscing elit.</p>
<p><strong>Core code:</strong></p>
<pre><code>.middle-demo-1{
height: 4em;
line-height: 4em;
overflow: hidden;
}</code></pre>
<dl>
<dt>Notes:</dt>
<dd>
<ol>
<li>I strongly recommend you use relative size in <span class="property">height</span> and <span class="property">line-height</span> property. <em>Why?</em> You can simply set the font size larger if your browser support it. When it gets large enough, you
will see the countainer is stretched and the <span class="property">height</span> is no longer equal to <span class="property">line-height</span> property, thus, the layout is messed up. Using relative size as <span class="property">em, ex</span> or <span
class="property">%</span> will let your countainer stretch with the content.</li>
<li><span class="property">overflow: hidden</span> is a must. <em>Why?</em> Same as above. Just ensure <span class="property">height</span> and <span class="property">line-height</span> are always equal.</li>
</ol>
</dd>
</dl>
<dl>
<dt>Pros:</dt>
<dd>
<ol>
<li>Fits in both <span class="property">block</span> elements and <span class="property">inline</span> elements.</li>
<li>Capable of all 5th-gen browsers.</li>
</ol>
</dd>
</dl>
<dl>
<dt>Cons:</dt>
<dd>
<ol>
<li>Text length is limited. Max with only <em>one</em> line.</li>
<li>Does not work well on none text contents such as images and objects.</li>
</ol>
</dd>
</dl>
<p class="snap-back"><a href="#bullet-0" title="Back top table of centents">Back</a></p>
<h2><a id="bullet-2">Case Two: Align multi-line container which does not have a fixed height</a></h2>
<p>In this case, we should simply set a pair of fixed equivalences to padding-top and padding-bottom attribute. It works on both IE and non-IE browsers.</p>
<p><strong>Sample:</strong></p>
<div class="demo" id="demo-2">
<p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Maecenas dignissim diam eu sem. Proin nunc ante, accumsan sollicitudin, sodales at, semper sed, ipsum. Etiam orci. Vestibulum magna lectus, venenatis nec, tempus ac, dictum vel, lorem.</p>
</div>
<p><strong>Core code:</strong></p>
<pre><code>.middle-demo-2{
padding-top: 24px;
padding-bottom: 24px;
}</code></pre>
<dl>
<dt>Pros:</dt>
<dd>
<ol>
<li>Fits in both <span class="property">block</span> elements and <span class="property">inline</span> elements.</li>
<li>Works on none text contents as fine as text contents.</li>
<li>Capable of all 5th-gen browsers. Might need a little tune-up for the box model bug of IE5 though.</li>
</ol>
</dd>
</dl>
<dl>
<dt>Cons:</dt>
<dd>
<ol>
<li>You can <em>not</em> assign height in this solution.</li>
</ol>
</dd>
</dl>
<p class="snap-back"><a href="#bullet-0" title="Back top table of centents">Back</a></p>
<h2><a id="bullet-3">Case Three: Simulating table layout in container with a fixed height</a></h2>
<p>CSS offers a set of very convenient display atrribute values called <span class="property">display: table</span>, <span class="property">display: table-row</span>, <span class="property">display: table-cell</span> and other display values begin with <span
class="property">table-</span>. It offers a way to simulate table layout in all elements. As a result, any element with <span class="property">display: table-cell</span>, <span class="property">vertical-align: middle</span> and a fixed height will display
exactly like a table cell.</p>
<p><strong>Sample:</strong> (You may not be able to view the effect on IE)</p>
<div class="demo" id="demo-3">
<div><div><p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Maecenas dignissim diam eu sem. Proin nunc ante, accumsan sollicitudin, sodales at, semper sed, ipsum. Etiam orci. Vestibulum magna lectus, venenatis nec, tempus ac, dictum vel, lorem.</p></div></div>
</div>
<p><strong>Core code:</strong></p>
<pre><code>.middle-demo-3{
display: table-cell;
height: 300px;
vertical-align: middle;
}</code></pre>
<dl>
<dt>Notes:</dt>
<dd>
<ol>
<li><span class="property">display: table-cell</span> must work with other elements with <span class="property">display: table</span> value sets in order to form a literal table. Or it might cause unexceptable bugs.</li>
<li>Sadly IE series (including the latest IE 7 beta) does not support any of <span class="property">display: table</span> values, so it won't work in IE.</li>
</ol>
</dd>
</dl>
<dl>
<dt>Pros:</dt>
<dd>
<ol>
<li>It has the most perfect render for <span class="property">vertical-align: middle</span> align.</li>
</ol>
</dd>
</dl>
<dl>
<dt>Cons:</dt>
<dd>
<ol>
<li>It only works in latest versions of non-IE browsers, such as Mozilla, Firefox, Netscape 8, Opera 8, and Safari.</li>
</ol>
</dd>
</dl>
<p class="snap-back"><a href="#bullet-0" title="Back top table of centents">Back</a></p>
<h2><a id="bullet-4">Case Four: IE's solution</a></h2>
<p>Since IE is a lame browser when it comes to ANYTHING, so you can't use the solution above simply because IE does not recognize it at all. However, <strong>there has been nothing yet you can not code with CSS if you have already coded it with table</strong>.
It even offers you possibility to layout your page that tables can not do!</p>
<p>So what's the solution for IE? Like what we always do: Hit IE's back with the BUGS it offers!</p>
<p><strong>Sample:</strong> (You are able to view the correct result only on IE)</p>
<div class="demo" id="demo-4">
<div><p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Maecenas dignissim diam eu sem. Proin nunc ante, accumsan sollicitudin, sodales at, semper sed, ipsum. Etiam orci. Vestibulum magna lectus, venenatis nec, tempus ac, dictum vel, lorem.</p></div>
</div>
<pre><code>.middle-demo-4{
height: 300px;
position: relative;
}
.middle-demo-4 div{
position: absolute;
top: 50%;
left: 0;
}
.middle-demo-4 div div{
position: relative;
top: -50%;
left: 0;
}</code></pre>
<p>See? Don't ask me why it worked. This hack is based on years of study in IE's own stupid layout model and it works very well, even in IE5 given the box width hack. I won't tell you the theory behind all these. It's my top secret, with which I solved a lot
of other cross browser layout problems, and I'm not going to share it with you. But you are free to use this IE hack It's kinda handful in most occassions.</p>
<dl>
<dt>Pros:</dt>
<dd>
<ol>
<li>The only perfect vertical align method in IE. It works even better then add automatic <span class="property">padding</span>s.</li>
</ol>
</dd>
</dl>
<dl>
<dt>Cons:</dt>
<dd>
<ol>
<li>After all it's a CSS hack. We can avoid it if not for IE.</li>
</ol>
</dd>
</dl>
<p class="snap-back"><a href="#bullet-0" title="Back top table of centents">Back</a></p>
<h2><a id="bullet-5">Case Five: A perfect compounded sample</a></h2>
<p>Here's the perfect compounded solution on vertical centering that works on almost all latest browsers.</p>
<p><strong>Sample:</strong> (This works on almost all browsers)</p>
<div class="demo" id="demo-5">
<div><p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Maecenas dignissim diam eu sem. Proin nunc ante, accumsan sollicitudin, sodales at, semper sed, ipsum. Etiam orci. Vestibulum magna lectus, venenatis nec, tempus ac, dictum vel, lorem.</p></div>
</div>
<p>I'm not going to give you the full code of this one. But it's not difficult to write it yourself, with the solution of case 3 and 4, and a little knowledge in IE/non-IE CSS hackers. And actually the hacking style is not limited, so I don't want to force
you to use my hacking style neither. Stop being lazy, and write the code yourself!</p>
<dl>
<dt>Browser capability:</dt>
<dd>
<ol>
<li>Works on: IE6+, Mozilla 1.5+, Netscape Navigator 7+, Opera 7+, Firefox 1.0+ and Safari 1.0+</li>
<li>On IE5, you might need to apply the box model hacker to make the height correct.</li>
<li>Untested: WebOmini, Konqueror.</li>
<li>Fails on: Other browsers not reffered in the list above.</li>
</ol>
</dd>
</dl>
<p class="snap-back"><a href="#bullet-0" title="Back top table of centents">Back</a></p>
<p class="copyright">Copyright © Spenser Lee, 2006 Liberty Studio</p>
</div>
</body>
</html>