Function do_substitute_chunk 

fn do_substitute_chunk<'a>(
    css: &'a str,
    start: usize,
    end: usize,
    first_token_type: TokenSerializationType,
    last_token_type: TokenSerializationType,
    url_data: &UrlExtraData,
    substitution_functions: &'a ComputedSubstitutionFunctions,
    stylist: &Stylist,
    computed_context: &Context<'_>,
    references: &mut Peekable<Iter<'_, SubstitutionFunctionReference>>,
    attribute_tracker: &mut AttributeTracker<'_>,
) -> Result<Substitution<'a>, ()>