問(wèn)題描述

如圖 這是我在刷面試題得時(shí)候 碰見(jiàn)的 想問(wèn)一下大神 有什么具體的區(qū)別？反正我寫(xiě)的時(shí)候 是沒(méi)有發(fā)現(xiàn) 至于作用我還是知道的

問(wèn)題解答

單冒號(hào)(:)用于CSS3偽類，雙冒號(hào)(::)用于CSS3偽元素。W3C關(guān)于CSS3選擇器的規(guī)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.

簡(jiǎn)單翻譯一下，大意就是，偽元素由雙冒號(hào)和偽元素名稱組成。雙冒號(hào)是在當(dāng)前規(guī)范中引入的，用于區(qū)分偽類和偽元素。但是偽類兼容現(xiàn)存樣式，瀏覽器需要同時(shí)支持舊的偽類，比如:first-line、:first-letter、:before、:after等。

那么現(xiàn)在就可以完整的回答標(biāo)題中的問(wèn)題了，對(duì)于CSS2之前已有的偽元素，比如:before，單冒號(hào)和雙冒號(hào)的寫(xiě)法::before作用是一樣的。

所以，如果你的網(wǎng)站只需要兼容webkit、firefox、opera等瀏覽器，建議對(duì)于偽元素采用雙冒號(hào)的寫(xiě)法，如果不得不兼容IE瀏覽器，還是用CSS2的單冒號(hào)寫(xiě)法比較安全。

這種問(wèn)題百度就知道了

參見(jiàn):

偽元素 https://developer.mozilla.org/zh-CN/docs/Web/CSS/Pseudo-elements

偽類 https://developer.mozilla.org/zh-CN/docs/Web/CSS/Pseudo-classes