CSS规则的执行顺序（转）

你对CSS规则的执行顺序是否了解，这里和大家分享一下，若两条规则具有相同的权值、起源及特殊性，那在样式表中最后出现的规则优先。

1、CSS规则之特殊性

首先来看一下这个例子将会发生的情形：

<style type="text/css">
    .grape
    {
        color: Blue;
    }
    H1
    {
        color: Red;
    }
</style>
<h1 class="grape">
    Meerkat <em>Central </em>
</h1>

H1和.grape都匹配上面的H1元素，那么到底应该使用哪一个呢？实践证明.grape是正确答案，把句子显示为蓝色。根据规范，一般的HTML元素选择符(H1,P等)具有特殊性：

1、类选择符具有特殊性10，ID选择符具有特殊性100，值越大，权重就越大，越优先。

<style type="text/css">
    H1
    {
        color: Red;
    }
    /*特殊性=1*/

    PEM
    {
        color: Blue;
    }
    /*特殊性=2*/

    .grape
    {
        color: Fuchsia;
    }
    /*特殊性=10*/

    P.bright
    {
        color: Yellow;
    }
    /*特殊性=11*/

    P.brightEM.dark
    {
        color: Gray;
    }
    /*特殊性=12*/

    #ID01
    {
        color: Red;
    }
    /*特殊性=100*/
</style>

2、CSS规则之继承

在特殊性的框架下，被继承的值具有特殊性0，也就是说任何显式声明的规则将会覆盖其继承样式，即便这条规则具有多高的权重。

<style type="text/css">
 #ID01{color:Red;}
    /*特殊性=100*/
    EM{color:Gray;}
    /*特殊性=1*/
</style>
<h1 id="ID01">
    Meerkat<em> Central</em>
</h1>

虽然ID选择符特殊性最高，但由于在特殊性的框架下，继承值只有特殊性0，因些Central会显示为Gray颜色。

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3、CSS规则之STYLE元素

还有sytle元素在CSS下权值定义为100，尽管ID选择也一样，实际上style元素比ID具有更高的特殊性。

<style type="text/css">
 　　#ID01, EM{color:Gray;}
</style>
<h1 id="ID01">
    Meerkat<em style="color:red;"> Central</em>
</h1>

会显示为red颜色。

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4、CSS规则之重要性(!important)

!important具最高特特性比如说1000，因此!important规则会覆盖内联STYLE属性的内容。程序代码

<style type="text/css">
H1{color:red!important;}
</style>
<H1 style="color:black;">MeerkatCentral!</H1>

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一种特殊情形

<style type="text/css">
P#warn
    {
        color: Red !important;
    }
    EM
    {
        color: Black;
    }
</style>
<p id="warn">
    Thistextisred,but<em> emphasizedtextisblack.</em>
</p>

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虽然定义!important最高特殊性，但句子并没有全部显示为RED红色，为什么呢？

也许我们得回头看看前面的规则，在前面的第二点继承中提“在特殊性的框架下，继承值只有特殊性0。”

因此,即便定义!important，继承里的特殊性也只有0，所以显示为特殊性为1的EM规则。

权重顺序为：继承=>HTML普通选择符=>类选择符=>style元素=>!important

5、CSS规则之层叠

1)若两条规则具有相同的权值、起源及特殊性，那在样式表中最后出现的规则优先。

2)任何位于文档中的规则都比引入的规则优先。