Implement the ::backdrop pseudo-element for modal <dialog>.

Source/core/dom/PseudoElement.cpp

Index: Source/core/dom/PseudoElement.cpp
diff --git a/Source/core/dom/PseudoElement.cpp b/Source/core/dom/PseudoElement.cpp
index 468883b63cc6996a0e213fcab5ba42a1f867b522..519fe880dc94600ebb0dfa5da56e5c9e9be5dfad 100644
--- a/Source/core/dom/PseudoElement.cpp
+++ b/Source/core/dom/PseudoElement.cpp
@@ -81,7 +81,7 @@ void PseudoElement::attach(const AttachContext& context)
     Element::attach(context);
 
     RenderObject* renderer = this->renderer();
-    if (!renderer || !renderer->style()->regionThread().isEmpty())
+    if (!renderer || !renderer->style()->regionThread().isEmpty() || renderer->style()->styleType() == BACKDROP)

[esprehn, 2013/07/03 23:44:29]

Lets remove this and just add an early return below this line:

if (style()->styleType() != BEFORE && style()->styleType() != AFTER)
  return;

[falken, 2013/07/04 08:20:12]

Done. Good idea. But we need the !style->regionThread().isEmpty() check still.
It turns out you can have :before or :after and be in a region, in which case
the content shouldn't be generated
(fast/regions/content-flowed-into-pseudo-regions.html)

         return;
 
     RenderStyle* style = renderer->style();