Introduce :-internal-shadow-host-has-appearance pseudo class, and apply it to METER element.

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

Index: third_party/WebKit/Source/core/css/SelectorChecker.cpp
diff --git a/third_party/WebKit/Source/core/css/SelectorChecker.cpp b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
index 3416c2386789fce0db221a12c9e35cd4d44c2431..f50a13f3b07d5235481b1691ae159a071afb1895 100644
--- a/third_party/WebKit/Source/core/css/SelectorChecker.cpp
+++ b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
@@ -917,6 +917,14 @@ bool SelectorChecker::checkPseudoClass(const SelectorCheckingContext& context, M
         return m_isUARule && matchesSpatialNavigationFocusPseudoClass(element);
     case CSSSelector::PseudoListBox:
         return m_isUARule && matchesListBoxPseudoClass(element);
+    case CSSSelector::PseudoHostHasAppearance:
+        if (!m_isUARule)

[esprehn, 2016/05/09 19:39:50]

this should check that containingShadowRoot()->type() is a UserAgentShadowRoot.

[tkent, 2016/05/10 04:29:29]

Done.

+            return false;
+        if (!element.shadowHost())
+            return false;
+        if (!element.shadowHost()->computedStyle())

[esprehn, 2016/05/09 19:39:50]

Don't call computedStyle repeatedly, save to a local and null check that.

[tkent, 2016/05/10 04:29:29]

Done.

+            return false;
+        return element.shadowHost()->computedStyle()->hasAppearance();
     case CSSSelector::PseudoWindowInactive:
         if (!context.hasSelectionPseudo)
             return false;
@@ -960,11 +968,12 @@ bool SelectorChecker::checkPseudoElement(const SelectorCheckingContext& context,
             return false;
         }
     case CSSSelector::PseudoWebKitCustomElement:
-        {
-            if (ShadowRoot* root = element.containingShadowRoot())
-                return root->type() == ShadowRootType::UserAgent && element.shadowPseudoId() == selector.value();
-            return false;
+        if (ShadowRoot* root = element.containingShadowRoot()) {
+            if (selector.value().startsWith("-webkit-internal-") && !m_isUARule)

[esprehn, 2016/05/09 19:39:50]

why do we need this? We should just make the pseudo only apply inside a UA
shadow.

[tkent, 2016/05/10 04:29:29]

We should avoid to expose new vendor-specific pseudo elements.

+                return false;
+            return root->type() == ShadowRootType::UserAgent && element.shadowPseudoId() == selector.value();
         }
+        return false;
     case CSSSelector::PseudoSlotted:
         {
             SelectorCheckingContext subContext(context);