Add a UA style, "display: contents", for <slot>

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

Index: third_party/WebKit/Source/core/css/CSSDefaultStyleSheets.cpp
diff --git a/third_party/WebKit/Source/core/css/CSSDefaultStyleSheets.cpp b/third_party/WebKit/Source/core/css/CSSDefaultStyleSheets.cpp
index 1a2b48ee9bd2eb370ddb8d62904f4f479509865d..77fb48b80f86360cf8573b64070b75660cbd1d1e 100644
--- a/third_party/WebKit/Source/core/css/CSSDefaultStyleSheets.cpp
+++ b/third_party/WebKit/Source/core/css/CSSDefaultStyleSheets.cpp
@@ -37,6 +37,7 @@
 #include "core/dom/Fullscreen.h"
 #include "core/html/HTMLAnchorElement.h"
 #include "core/html/HTMLHtmlElement.h"
+#include "core/html/HTMLSlotElement.h"
 #include "core/layout/LayoutTheme.h"
 #include "platform/PlatformResourceLoader.h"
 #include "wtf/LeakAnnotations.h"
@@ -159,6 +160,15 @@ bool CSSDefaultStyleSheets::ensureDefaultStyleSheetsForElement(
     changedDefaultStyle = true;
   }
 
+  if (isHTMLSlotElement(element) &&
+      RuntimeEnabledFeatures::cssDisplayContentsEnabled() &&
+      !m_slotStyleSheet) {

[kochi, 2016/11/01 06:43:13]

If the reason for having a separate UA stylesheet file only for
<slot> element is to gate the behavior by the runtime flag, could
you add a TODO for merging slot.css into html.css once
CSSDisplayContents flag gets enabled?

[rune, 2016/11/01 07:38:17]

I think we might as well add it to html.css. When display:contents is not
supported, we will end up with an empty rule for slot, which is OK I think.
Alternatively, you can conditionally add the slot.css source to the defaultRules
string in the constructor. Having it here, means we'll get a re-collection of
document rule features when we see a slot element the first time. I think it's
better to parse the slot rule unconditionally.

+    m_slotStyleSheet = parseUASheet(loadResourceAsASCIIString("slot.css"));
+    m_defaultStyle->addRulesFromSheet(slotStyleSheet(), screenEval());
+    m_defaultPrintStyle->addRulesFromSheet(slotStyleSheet(), printEval());
+    changedDefaultStyle = true;
+  }
+
   ASSERT(!m_defaultStyle->features().hasIdsInSelectors());
   ASSERT(m_defaultStyle->features().siblingRules.isEmpty());
   return changedDefaultStyle;
@@ -189,6 +199,7 @@ DEFINE_TRACE(CSSDefaultStyleSheets) {
   visitor->trace(m_mathmlStyleSheet);
   visitor->trace(m_mediaControlsStyleSheet);
   visitor->trace(m_fullscreenStyleSheet);
+  visitor->trace(m_slotStyleSheet);
 }
 
 }  // namespace blink