1.层次选择器
通过HTML的DOM元素间的层次关系获取元素，层次关系包括后代、父子、相邻兄弟和通用兄弟，通过其中某类关系可以方便快捷地选定需要的元素
2.语法
3.兼容性
IE7以及以上版本
4.后代选择器
也称包含选择器，作用：可以选择某元素的后代元素
eg：【E F】，选中F元素，这里不管F元素是E元素的子元素、孙辈元素或者更深层次的关系，都将被选中
注：后代选择器两选择符之间必须以空格隔开，中间不能有任何其他符号插入
5.子元素选择器
只能选择某元素的子元素，仅仅选中子元素而已
范围比后代选择器小
6.相邻兄弟选择器
可以选择紧接在另一个元素后的元素，它们具有一个相同的父元素（之后的元素，而不是之前的元素）
7.通用兄弟选择器
是CSS3新增加的，用于选择某元素后面的所有兄弟元素，和相邻兄弟选择器类似，都在同一个父元素之中，范围比相邻兄弟选择器广
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