Function apply_declarations 

pub fn apply_declarations<'decls, E, I>(
    stylist: &Stylist,
    pseudo: Option<&PseudoElement>,
    rules: &StrongRuleNode,
    guards: &StylesheetGuards<'_>,
    iter: I,
    parent_style: Option<&ComputedValues>,
    layout_parent_style: Option<&ComputedValues>,
    first_line_reparenting: FirstLineReparenting<'_>,
    try_tactic: &PositionTryFallbacksTryTactic,
    cascade_mode: CascadeMode<'_, '_>,
    cascade_input_flags: ComputedValueFlags,
    included_cascade_flags: RuleCascadeFlags,
    rule_cache: Option<&RuleCache>,
    rule_cache_conditions: &mut RuleCacheConditions,
    element: Option<E>,
    tree_counting_caches: &mut TreeCountingCaches,
) -> Arc<ComputedValues>
where
    E: TElement,
    I: Iterator<Item = (&'decls PropertyDeclaration, CascadePriority)>,

NOTE: This function expects the declaration with more priority to appear first.