CSS

伪元素与伪类

2017-05-31  本文已影响0人  Vampire618

伪元素种类:

伪类种类:

      伪元素和伪类之所以这么容易混淆,是因为他们的效果类似而且写法相仿,但实际上css3为了区分两者,已经明确规定了伪类用一个冒号来表示,而伪元素则用两个冒号来表示。但因为兼容性的问题,所以现在大部分还是统一的单冒号。

伪元素

1.first-line伪元素

first-line伪元素对段落内容的第一个格式化应用特殊样式。注意,第一行的长度取决于很多因素,包括页面宽度,字体大小等等。选择器"p:first-line"不会匹配任何真实的HTML元素,而是通过UA(也就是浏览器)给第一行加上 虚拟标签序列(fictional tag sequence),然后匹配这个东西。

比如:这段文本

<p>This is a somewhat long HTML paragraph that

will be broken into several

lines. The first line will be identified

by a fictional tag sequence. The other lines

will be treated as ordinary lines in the

paragraph.</p>

会被UA加上虚拟标签序列,像下面这样:

<p><p:first-line>This is a somewhat long HTML paragraph that</p:first-line>

will be broken into several

lines. The first line will be identified

by a fictional tag sequence. The other lines

will be treated as ordinary lines in the

paragraph.</p>

:first-line伪元素只能用于块级容器元素

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