In bug 238345 I plan to sort deferred properties after normal properties, since they cascade in a special way.
Something similar could be done for shorthands.

StyleBuilder.cpp does something like

    applyProperties(firstCSSProperty, lastHighPriorityProperty);
    applyProperties(firstLowPriorityProperty, lastCSSProperty);
    
    inline void Builder::applyPropertiesImpl(int firstProperty, int lastProperty)
    {
        for (int id = firstProperty; id <= lastProperty; ++id) {
            CSSPropertyID propertyID = static_cast<CSSPropertyID>(id);
            if (!m_cascade.hasProperty(propertyID))
                continue;

Currently, shorthand properties are mixed among longhands, so they are iterated, but hasProperty() will be false.
Then they are wasting some memory in PropertyCascade.h:

    Property m_properties[numCSSProperties + 2];
    std::bitset<numCSSProperties + 2> m_propertyIsPresent;

So I think CSSPropertyID could be sorted as such:

 1. Special meaning (CSSPropertyInvalid and CSSPropertyCustom)
 2. High priority longhands
 3. Low priority longhands
 4. Deferred longhands
 5. Shorthands

About a 15-20% of the properties seem to be shorthands, so it may be worth trying.