Represents a CSS pseudo-class such as :active, :focus, or :nthChild(odd).
Pseudo classes are appended to selectors using the % operator. They are chainable as well meaning
the following is possible:
a %active %hover becomes a:active:hover.
Special abbreviations exist such as "odd" for ":nthChild(odd)" and "even" for :nthChild(even).
||Only here to restrict the DSL so that pseudo-class is used properly.|