Function selectors::parser::parse_compound_selector

fn parse_compound_selector<'i, 't, P, Impl>(
    parser: &P,
    state: &mut SelectorParsingState,
    input: &mut Parser<'i, 't>,
    builder: &mut SelectorBuilder<Impl>,
) -> Result<bool, ParseError<'i, P::Error>>
where
    P: Parser<'i, Impl = Impl>,
    Impl: SelectorImpl,

simple_selector_sequence : [ type_selector | universal ] [ HASH | class | attrib | pseudo | negation ]* | [ HASH | class | attrib | pseudo | negation ]+

Err(()) means invalid selector. Ok(true) is an empty selector