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

Source/core/css/RuleFeature.cpp

Index: Source/core/css/RuleFeature.cpp
diff --git a/Source/core/css/RuleFeature.cpp b/Source/core/css/RuleFeature.cpp
index 75af986dfdf2af2346bf9c8d28c999733361c3c0..4f2a14b482c99e83407c64b7a97c0c9e44176ba6 100644
--- a/Source/core/css/RuleFeature.cpp
+++ b/Source/core/css/RuleFeature.cpp
@@ -126,16 +126,37 @@ RuleFeatureSet::InvalidationSetMode RuleFeatureSet::invalidationSetModeForSelect
         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.
+                    if (subSelectorMode == UseSubtreeStyleChange)
                         return foundCombinator ? UseLocalStyleChange : UseSubtreeStyleChange;

[chrishtr, 2014/07/11 18:14:08]

Please explain in the comment why it is sometimes allowed to return
UseLocalStyleChange.

[rune, 2014/07/11 21:02:57]

I have to investigate why the _UseSubtreeStyleChange_ is still needed. The
returned value from this method is the input to the collectFeaturesFromSelector
method which does the fallback for adding invalidation set features.
UseLocalStyleChange there means just create invalidation sets for the features
found in the rightmost compound to do a LocalStyleChange when any of those
features change, but start to use SubtreeStyleChange once we go past the first
combinator. So, if the features in the rightmost compound are ok, we can clearly
UseLocalStyleChange. I can't think of what type of selector found in the
rightmost compound selector will require a SubtreeStyleChange for any of its
features.

[rune, 2014/07/11 21:49:37]

Done.

-                    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)) {