Function compute_value 

pub fn compute_value(
    css: &str,
    url_data: &UrlExtraData,
    registration: &Descriptors,
    computed_context: &Context<'_>,
    attr_taint: AttrTaint,
    property_id: Option<PropertyIdRef<'_>>,
) -> Result<ComputedValue, ()>

Computes css against the registration of a custom property.