[css3-selectors] minor question about :not()

in 6.6.7:
"The negation pseudo-class, :not(X), is a functional notation taking a 
simple selector (excluding the negation pseudo-class itself and 
pseudo-elements) as an argument."

so, :not(:pseudo-element) - allowed by formal grammar -
is invalid selector or "useless" selector, as foo:not(bar) ?