关于选择器优先级的误解[刷新三观，深入理解选择器优先级]

unsigned CSSSelector::specificity() const
{
    // make sure the result doesn‘t overflow
    static const unsigned maxValueMask = 0xffffff; // 整个选择器的最大值，十进制表示：idMask + classMask + elementMak = 16777215
    static const unsigned idMask = 0xff0000; // ID选择器的最大值，十进制表示：(16*16+16)*16^4=16711680
    static const unsigned classMask = 0xff00; // class（伪类、类）选择器的最大值，十进制表示：(16*16+16)*16^2=65280
    static const unsigned elementMask = 0xff; // 元素选择器的最大值，十进制表示：16*16+16=255

    if (isForPage())
        return specificityForPage() & maxValueMask;

    unsigned total = 0;
    unsigned temp = 0;

    for (const CSSSelector* selector = this; selector; selector = selector->tagHistory()) {
        temp = total + selector->specificityForOneSelector();
        // Clamp each component to its max in the case of overflow.
        if ((temp & idMask) < (total & idMask)) // 判断是否为ID选择器
            total |= idMask; // 保证ID选择器的同类叠加不会超过ID选择器的总最大值，下同
        else if ((temp & classMask) < (total & classMask))
            total |= classMask;
        else if ((temp & elementMask) < (total & elementMask))
            total |= elementMask;
        else
            total = temp;
    }
    return total;
}

inline unsigned CSSSelector::specificityForOneSelector() const
{
    // FIXME: Pseudo-elements and pseudo-classes do not have the same specificity. This function
    // isn‘t quite correct.
    switch (m_match) {
    case Id:
        return 0x10000; // ID选择器权重

    case PseudoClass:
        // FIXME: PsuedoAny should base the specificity on the sub-selectors.
        // See http://lists.w3.org/Archives/Public/www-style/2010Sep/0530.html
        if (pseudoClassType() == PseudoClassNot && selectorList())
            return selectorList()->first()->specificityForOneSelector();
        FALLTHROUGH;
    case Exact:
    case Class:
    case Set:
    case List:
    case Hyphen:
    case PseudoElement:
    case Contain:
    case Begin:
    case End:
        return 0x100; // class选择器权重

    case Tag:
        return (tagQName().localName() != starAtom) ? 1 : 0; // 元素选择器权重
    case Unknown:
        return 0;
    }
    ASSERT_NOT_REACHED();
    return 0;
}

int32_t nsCSSSelector::CalcWeightWithoutNegations() const
 {
   int32_t weight = 0;
 
 #ifdef MOZ_XUL
   MOZ_ASSERT(!(IsPseudoElement() &&
                PseudoType() != nsCSSPseudoElements::ePseudo_XULTree &&
                mClassList),
              "If non-XUL-tree pseudo-elements can have class selectors "
              "after them, specificity calculation must be updated");
 #else
   MOZ_ASSERT(!(IsPseudoElement() && mClassList),
              "If pseudo-elements can have class selectors "
              "after them, specificity calculation must be updated");
 #endif
   MOZ_ASSERT(!(IsPseudoElement() && (mIDList || mAttrList)),
              "If pseudo-elements can have id or attribute selectors "
              "after them, specificity calculation must be updated");
 
   if (nullptr != mCasedTag) {
     weight += 0x000001;
   }
   nsAtomList* list = mIDList;
   while (nullptr != list) {
     weight += 0x010000;
     list = list->mNext;
   }
   list = mClassList;
 #ifdef MOZ_XUL
   // XUL tree pseudo-elements abuse mClassList to store some private
   // data; ignore that.
   if (PseudoType() == nsCSSPseudoElements::ePseudo_XULTree) {
     list = nullptr;
   }
 #endif
   while (nullptr != list) {
     weight += 0x000100;
     list = list->mNext;
   }
   // FIXME (bug 561154):  This is incorrect for :-moz-any(), which isn‘t
   // really a pseudo-class.  In order to handle :-moz-any() correctly,
   // we need to compute specificity after we match, based on which
   // option we matched with (and thus also need to try the
   // highest-specificity options first).
   nsPseudoClassList *plist = mPseudoClassList;
   while (nullptr != plist) {
     weight += 0x000100;
     plist = plist->mNext;
   }
   nsAttrSelector* attr = mAttrList;
   while (nullptr != attr) {
     weight += 0x000100;
     attr = attr->mNext;
   }
   return weight;
 }
 
 int32_t nsCSSSelector::CalcWeight() const
 {
   // Loop over this selector and all its negations.
   int32_t weight = 0;
   for (const nsCSSSelector *n = this; n; n = n->mNegations) {
     weight += n->CalcWeightWithoutNegations();
   }
   return weight;
 }