Function get_attr_value_for_cycle_resolution 

fn get_attr_value_for_cycle_resolution(
    name: &Atom,
    attribute_data: &AttributeData,
    url_data: &UrlExtraData,
    attribute_tracker: &mut AttributeTracker<'_>,
) -> Result<ComputedValue, ()>

Attribute values may reference other substitution functions we may need to process. See step 6: https://drafts.csswg.org/css-values-5/#attr-substitution