https://www.w3.org/TR/css3-selectors/#selector-syntax
以上链接说：
A selector is a chain of one or more sequences of simple selectors
separated by combinators.
它还说：
A simple selector is either a type selector,universal selector,
attribute selector,class selector,ID selector,or pseudo-class.
关于如何使用伪元素,它说：
One pseudo-element may be appended to the last sequence of simple
selectors in a selector.
和
Only one pseudo-element may appear per selector,and if present it
must appear after the sequence of simple selectors that represents the
subjects of the selector.
那么这是否意味着伪元素只能是“有效”选择器的后缀而不应该单独使用？
解决方法
does that mean that a pseudo-element can only be a suffix to a “valid”
selector and should not be used alone?
你的结论不正确,因为universal selector *可以省略.
If a universal selector represented by
*[…] is immediately
followed by a 07001,then the*may be omitted and the
universal selector’s presence implied.
所以你可以单独使用一个伪元素,例如::之前,因为在引擎盖下它将被视为* ::之前.
::before {
content: 'Hello!';
}
