elements be used alone in CSS?
According to W3C, the definition of a selector does not cover a pseudo-element: https://www.w3.org/TR/css3-selectors/#selector-syntax
The above link says:
A selector is a chain of one or more sequences of simple selectors separated by combinators.
and it also says:
A simple selector is either a type selector, universal selector, attribute selector, class selector, ID selector, or pseudo-class.
Regarding how a pseudo-element should be used, it says:
One pseudo-element may be appended to the last sequence of simple selectors in a selector.
and
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.
So does that mean that a pseudo-element can only be a suffix to a "valid" selector and should not be used alone?
does that mean that a pseudo-element can only be a suffix to a "valid" selector and should not be used alone?
Your conclusion is not true, because the universal selector * can be omitted.
If a universal selector represented by * [...] is immediately followed by a pseudo-element, then the * may be omitted and the universal selector's presence implied.
So you can use a pseudo-element alone, eg ::before , because under the hood it will be treated like *::before .
::before {
content: 'Hello!';
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