Function parse_declaration_value 

fn parse_declaration_value(
    input: &mut Parser<'_>,
    input_start: SourcePosition,
    namespaces: Option<&FxHashMap<Prefix, Namespace>>,
    references: &mut References,
    missing_closing_characters: &mut String,
) -> Result<(TokenSerializationType, TokenSerializationType), ParseError>

https://drafts.csswg.org/css-syntax-3/#typedef-declaration-value