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

third_party/WebKit/Source/core/dom/Element.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003-2011, 2013, 2014 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 432 matching lines @@
 
     void setPointerCapture(int pointerId, ExceptionState&);
     void releasePointerCapture(int pointerId, ExceptionState&);
 
     String textFromChildren();
 
     virtual String title() const { return String(); }
     virtual String defaultToolTip() const { return String(); }
 
     virtual const AtomicString& shadowPseudoId() const;
+    // The specified ID string must start with "-webkit-". Also, if it starts
+    // with "-webkit-internal-", selectors including the pseudo ID is usable

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

just -internal, not -webkit-internal. We don't add more -webkits. :)

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

Done.
PseudoWebKitCustomElement needs to start with "-webkit-".  New patch introduces
PseudoBlinkInternalElement, starting with "-internal-".

+    // only in UA stylesheet.
     void setShadowPseudoId(const AtomicString&);
 
     LayoutSize minimumSizeForResizing() const;
     void setMinimumSizeForResizing(const LayoutSize&);
 
     virtual void didBecomeFullscreenElement() { }
     virtual void willStopBeingFullscreenElement() { }
 
     // Called by the parser when this element's close tag is reached,
     // signaling that all child tags have been parsed and added.
@@ skipping 498 matching lines @@
     static T* create(const QualifiedName&, Document&)
 #define DEFINE_ELEMENT_FACTORY_WITH_TAGNAME(T) \
     T* T::create(const QualifiedName& tagName, Document& document) \
     { \
         return new T(tagName, document); \
     }
 
 } // namespace blink
 
 #endif // Element_h