:before和::before的区别

在一次项目中,有一次要用到::selection伪元素,然后开发同学问我,CSS中一个冒号和两个冒号有神马区别?

这好像真的是个问题,或许很多前端同学对此都有疑惑,查了些资料,证实了下两个符号的区别,简而言之:单冒号(:)用于CSS3伪类,双冒号(::)用于CSS3伪元素

W3C关于CSS3选择器的规范中有一段描述:

A pseudo-element is made of two colons (::) followed by the name of the pseudo-element.
This :: notation is introduced by the current document in order to establish a discrimination between pseudo-classes and pseudo-elements. For compatibility with existing style sheets, user agents must also accept the previous one-colon notation for pseudo-elements introduced in CSS levels 1 and 2 (namely, :first-line, :first-letter, :before and :after). This compatibility is not allowed for the new pseudo-elements introduced in CSS level 3.

简单翻译一下,大意就是,伪元素由双冒号和伪元素名称组成。双冒号是在当前规范中引入的,用于区分伪类和伪元素。不过浏览器需要同时支持旧的已经存在的伪元素写法,比如:first-line、:first-letter、:before、:after等,而新的在CSS3中引入的伪元素则不允许再支持旧的单冒号的写法。

那么现在就可以完整的回答标题中的问题了,对于CSS2之前已有的伪元素,比如:before,单冒号和双冒号的写法::before作用是一样的

所以,如果你的网站只需要兼容webkit、firefox、opera等浏览器,建议对于伪元素采用双冒号的写法,如果不得不兼容IE浏览器,还是用CSS2的单冒号写法比较安全。