All selectors in :-webkit-any must have an invalidation set feature.

Source/core/css/RuleFeature.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 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/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 108 matching lines @@
 // This method is somewhat conservative in what it accepts.
 RuleFeatureSet::InvalidationSetMode RuleFeatureSet::invalidationSetModeForSelector(const CSSSelector& selector)
 {
     bool foundCombinator = false;
     bool foundIdent = false;
     for (const CSSSelector* component = &selector; component; component = component->tagHistory()) {
 
         if (component->match() == CSSSelector::Class || component->match() == CSSSelector::Id
             || (component->match() == CSSSelector::Tag && component->tagQName().localName() != starAtom)
             || component->isAttributeSelector() || component->isCustomPseudoElement()) {
-            if (!foundCombinator)
+            if (!foundCombinator) {
+                // We have found an invalidation set feature in the rightmost compound selector.
                 foundIdent = true;
+            }
         } else if (component->pseudoType() == CSSSelector::PseudoHost || component->pseudoType() == CSSSelector::PseudoAny) {
             if (const CSSSelectorList* selectorList = component->selectorList()) {
+                bool foundUniversal = false;
                 for (const CSSSelector* selector = selectorList->first(); selector; selector = CSSSelectorList::next(*selector)) {
-                    InvalidationSetMode hostMode = invalidationSetModeForSelector(*selector);
-                    if (hostMode == UseSubtreeStyleChange)
+                    // Find the invalidation set mode for each of the selectors in the selector list
+                    // of a :not(), :host(), etc. For instance, ".x :-webkit-any(.a, .b)" yields an
+                    // AddFeatures mode for both ".a" and ".b". ":-webkit-any(.a, *)" yields AddFeatures
+                    // for ".a", but UseSubtreeStyleChange for "*". One sub-selector without invalidation
+                    // set features is sufficient to cause the selector to be a universal selector as far
+                    // the invalidation set is concerned.
+                    InvalidationSetMode subSelectorMode = invalidationSetModeForSelector(*selector);
+
+                    // The sub-selector contained something unskippable, fall back to whole subtree
+                    // recalcs in collectFeaturesFromSelector. subSelectorMode will return
+                    // UseSubtreeStyleChange since there are no combinators inside the selector list,
+                    // so translate it to UseLocalStyleChange if a combinator has been seen in the
+                    // outer context.
+                    //
+                    // FIXME: Is UseSubtreeStyleChange ever needed as input to collectFeaturesFromSelector?
+                    // That is, are there any selectors for which we need to use SubtreeStyleChange for
+                    // changing features when present in the rightmost compound selector?

[rune, 2014/08/08 14:35:50]

Yes, but only for :host-context() since :host-context() has an implicit
descendant selector. We can get rid of the whole mode thing if we handle the
implicit descendant combinator for :host-context() in
collectFeaturesFromSelector instead. But that's for another CL.

+                    if (subSelectorMode == UseSubtreeStyleChange)
                         return foundCombinator ? UseLocalStyleChange : UseSubtreeStyleChange;
-                    if (!foundCombinator && hostMode == AddFeatures)
-                        foundIdent = true;
+
+                    // We found no features in the sub-selector, only skippable ones (foundIdent was
+                    // false at the end of this method). That is a universal selector as far as the
+                    // invalidation set is concerned.
+                    if (subSelectorMode == UseLocalStyleChange)
+                        foundUniversal = true;
+                }
+                if (!foundUniversal && !foundCombinator) {
+                    // All sub-selectors contained invalidation set features and
+                    // we are in the rightmost compound selector.
+                    foundIdent = true;
                 }
             }
         } else if (!isSkippableComponentForInvalidation(*component)) {
             return foundCombinator ? UseLocalStyleChange : UseSubtreeStyleChange;
         }
         if (component->relation() != CSSSelector::SubSelector)
             foundCombinator = true;
     }
     return foundIdent ? AddFeatures : UseLocalStyleChange;
 }
@@ skipping 357 matching lines @@
     visitor->trace(siblingRules);
     visitor->trace(uncommonAttributeRules);
     visitor->trace(m_classInvalidationSets);
     visitor->trace(m_attributeInvalidationSets);
     visitor->trace(m_idInvalidationSets);
     visitor->trace(m_pseudoInvalidationSets);
     visitor->trace(m_styleInvalidator);
 }
 
 } // namespace WebCore