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CSS规则的执行顺序（转）
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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。
aaarticlea/png;base64,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" alt="" />
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。
aaarticlea/png;base64,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" alt="" />
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>
aaarticlea/png;base64,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" alt="" />将显示为RED颜色。
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>
aaarticlea/png;base64,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" alt="" />
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。
