Rephrased documentation about VTT, ::cue, and custom pseudo scopes.

third_party/WebKit/Source/core/css/resolver/StyleResolver.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 204 matching lines @@
   if (!m_styleSharingLists[depth])
     m_styleSharingLists[depth] = new StyleSharingList;
   return *m_styleSharingLists[depth];
 }
 
 void StyleResolver::clearStyleSharingList() {
   m_styleSharingLists.resize(0);
 }
 
 static inline ScopedStyleResolver* scopedResolverFor(const Element& element) {
-  // Ideally, returning element->treeScope().scopedStyleResolver() should be
-  // enough, but ::cue and custom pseudo elements like ::-webkit-meter-bar
-  // pierce through a shadow dom boundary, yet they are not part of boundary
-  // crossing rules. The assumption here is that these rules only pierce through
-  // one boundary and that the scope of these elements do not have a style
-  // resolver due to the fact that VTT scopes and UA shadow trees don't have
-  // <style> elements. This is backed up by the DCHECKs below.
+  // For normal elements, returning element->treeScope().scopedStyleResolver()
+  // is enough. Rules for ::cue and custom pseudo elements like
+  // ::-webkit-meter-bar pierce through a single shadow dom boundary and apply
+  // to elements in sub-scopes.
+  //
+  // An assumption here is that these elements belong to scopes without a
+  // ScopedStyleResolver due to the fact that VTT scopes and UA shadow trees
+  // don't have <style> or <link> elements. This is backed up by the DCHECKs
+  // below.
 
   TreeScope* treeScope = &element.treeScope();
   if (ScopedStyleResolver* resolver = treeScope->scopedStyleResolver()) {
     DCHECK(element.shadowPseudoId().isEmpty());
     DCHECK(!element.isVTTElement());
     return resolver;
   }
 
   treeScope = treeScope->parentTreeScope();
   if (!treeScope)
@@ skipping 1721 matching lines @@
 
 DEFINE_TRACE(StyleResolver) {
   visitor->trace(m_matchedPropertiesCache);
   visitor->trace(m_selectorFilter);
   visitor->trace(m_styleSharingLists);
   visitor->trace(m_document);
   visitor->trace(m_tracker);
 }
 
 }  // namespace blink