Implement ::placeholder CSS selector.

third_party/WebKit/Source/core/css/ElementRuleCollector.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
@@ skipping 178 matching lines @@
   ASSERT(matchRequest.ruleSet);
   ASSERT(m_context.element());
 
   Element& element = *m_context.element();
   const AtomicString& pseudoId = element.shadowPseudoId();
   if (!pseudoId.isEmpty()) {
     ASSERT(element.isStyledElement());
     collectMatchingRulesForList(
         matchRequest.ruleSet->shadowPseudoElementRules(pseudoId), cascadeOrder,
         matchRequest);
+    collectMatchingRulesForList(matchRequest.ruleSet->placeholderPseudoRules(),
+                                cascadeOrder, matchRequest);

[rune, 2016/11/21 08:52:33]

This means we'll try to match ::placeholder rules for all custom pseudo
elements, right? Should be guarded by if-test for placeholder pseudo id?

[tkent, 2016/11/21 09:23:39]

Oh, that's right.  Fixed.

   }
 
   if (element.isVTTElement())
     collectMatchingRulesForList(matchRequest.ruleSet->cuePseudoRules(),
                                 cascadeOrder, matchRequest);
   // Check whether other types of rules are applicable in the current tree
   // scope. Criteria for this:
   // a) the rules are UA rules.
   // b) matching tree boundary crossing rules.
   // c) the rules come from a shadow style sheet in the same tree scope as the
@@ skipping 162 matching lines @@
   return !m_matchedRules.isEmpty();
 }
 
 void ElementRuleCollector::addMatchedRulesToTracker(
     StyleRuleUsageTracker* tracker) const {
   for (auto matchedRule : m_matchedRules)
     tracker->track(matchedRule.ruleData()->rule());
 }
 
 }  // namespace blink