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—-非原创，摘录自 原文
优先权问题其实就是一个冲突解决的问题，当同一个元素（内容）被CSS选择符选中时，就要按照优先权取舍不同的CSS规则，这其中涉及到的问题其实很多。
首 先就是CSS规则的specificity（特殊性），CSS2.1有一套关于specificity的计算方式，用一个四位的数字符串（CSS2是三 位）来表示，最终specificity越高的规则越特殊，在优先级判定时也就越有优势。关于specificity的具体计算在各种情况下的数字加成有 如下一般规则：
RewriteRule
from: Apache2.2_chinese_manual
| 说明 | Defines rules for the rewriting engine |
|---|---|
| 语法 | RewriteRule Pattern Substitution |
| 作用域 | server config, virtual host, directory, .htaccess |
| 覆盖项 | FileInfo |
| 状态 | 扩展(E) |
| 模块 | mod_rewrite |
| 兼容性 | The cookie-flag is available in Apache 2.0.40 及以后的版本中可用 |
RewriteRule directive is the real rewriting workhorse. The directive can occur more than once. Each directive then defines one single rewriting rule. The definition order of these rules is important, because this order is used when applying the rules at run-time.
Pattern is a perl compatible regular expression which gets applied to the current URL. Here “current” means the value of the URL when this rule gets applied. This may not be the originally requested URL, because any number of rules may already have matched and made alterations to it.
Some hints about the syntax of regular expressions:
