選擇器的優先級關系到元素應用哪個樣式。在CSS2.1的規范（http://www.w3.org/TR/2009/CR-CSS2-20090908/cascade.html#specificity）中是這樣描述的：
- 如果聲明來自於“style”屬性，而不是帶有選擇器的規則，則記為 1，否則記為 0 (= a)（HTML元素的style屬性也是樣式規則，因為這些樣式規則沒有選擇器，因此記為a=1,b=0,c=0,d=0）
- 計算選擇器中 ID 屬性的個數 (= b)
- 計算選擇器中其他屬性(類、屬性選擇器)和偽類的個數 (= c)
- 計算選擇器中元素名稱和偽元素的個數 (= d)
將四個數字按 a-b-c-d 這樣連接起來（位於大數進制的數字系統中），構成選擇器的優先級。
在最新的Selector Level 3規范中：
- 計算選擇器中 ID 屬性的個數 (= a)
- 計算選擇器中其他屬性(類、屬性選擇器)和偽類的個數 (= b)
- 計算選擇器中元素名稱和偽元素的個數 (= c)
- 忽略通用選擇器*
問題：
1、選擇器的整體優先級如何計算，是像網上說的a*1000+b*100+c*10+d嗎？
unsigned CSSSelector::specificity() const { // make sure the result doesn't overflow static const unsigned maxValueMask = 0xffffff; // 整個選擇器的最大值，十進制表示：idMask + classMask + elementMak = 16777215 static const unsigned idMask = 0xff0000; // ID選擇器的最大值，十進制表示：(16*16+16)*16^4=16711680 static const unsigned classMask = 0xff00; // class（偽類、類）選擇器的最大值，十進制表示：(16*16+16)*16^2=65280 static const unsigned elementMask = 0xff; // 元素選擇器的最大值，十進制表示：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)) // 判斷是否為ID選擇器 total |= idMask; // 保證ID選擇器的同類疊加不會超過ID選擇器的總最大值，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選擇器權重 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選擇器權重 case Tag: return (tagQName().localName() != starAtom) ? 1 : 0; // 元素選擇器權重 case Unknown: return 0; } ASSERT_NOT_REACHED(); return 0; }
時間戳:2012-10-04 19:04:44 ( 20個月前)作者:commit-queue@webkit.org消息:選擇器特殊性類別溢出到高類別
https://bugs.webkit.org/show_bug.cgi?id=98295Patch by Tab Atkins <jackalmage@gmail.com> on 2012-10-04
Reviewed by Eric Seidel.這一次添加的補丁是為了對於CSS選擇器的特殊性添加溢出策略。
以前我們並不會檢測每個類別的特殊性溢出問題。原始的策略是：把每個類別存儲為一個字節（2^8=256），然后整體存在一個無符號整型數中。這樣的話就會導致256個同一類別的單選擇器等於1個高類別的選擇器。但是這違反了選擇器的特殊性規則，導致樣式規則排序問題。
Tests: /fast/selectors/specificity-overflow.html
- css/CSSSelector.cpp:
(WebCore::CSSSelector::specificity):
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; }
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>JS Bin</title>
</head>
<style>
*{font-size:40px;}
#test {font-size:12px !important;}
p {font-size:24px;}
</style>
<body>
<div id="test"><p>test text</p></div>
</body>
在所有瀏覽器中文字都會應用p {font-size:24px;}。如果把這句去掉的話，就會應用*{font-size:40px;}，*包括p。（繼承的樣式沒有優先級）
結論：
1、優先級計算時跨級相加應注意溢出問題；
2、優先級計算不包括inline style和!important；
3、優先級計算只有同一類別才具有可比性（一般也不會有人定義超出255個的同一選擇器）。
I am currently using the book CSS Mastery: Advanced Web Standards Solutions.
Chapter 1, page 16 says:
To calculate how specific a rule is, each type of selector is assigned a numeric value. The specificity of a rule is then calculated by adding up the value of each of its selectors. Unfortunately, specificity is not calculated in base 10 but a high, unspecified, base number. This is to ensure that a highly specific selector, such as an ID selector, is never overridden by lots of less specific selectors, such as type selectors.