No need to re-parse empty set of injected stylesheets.

Source/core/dom/StyleEngine.cpp

Index: Source/core/dom/StyleEngine.cpp
diff --git a/Source/core/dom/StyleEngine.cpp b/Source/core/dom/StyleEngine.cpp
index 1e9ff7b4f74627a83ced71d20b498cbe8a0ac18a..926d2c8e6fdceaeb9d9513cb801d151e90c817ee 100644
--- a/Source/core/dom/StyleEngine.cpp
+++ b/Source/core/dom/StyleEngine.cpp
@@ -231,6 +231,12 @@ void StyleEngine::invalidateInjectedStyleSheetCache()
     document().styleResolverChanged();
 }
 
+void StyleEngine::compatibilityModeChanged()
+{
+    if (!m_injectedAuthorStyleSheets.isEmpty())

[esprehn, 2014/08/19 11:30:59]

Is this correct? If the cache is empty why don't we need to recompute it and set
the dirty bit in there?

[rune, 2014/08/19 13:31:16]

The cache is the set of injected stylesheet which apply to the url of the
document for this style engine. That cache needs to be updated when the list of
injected stylesheets change since there might be stylesheets in the cache which
are removed from the global list, or newly added injected stylesheets which may
match the url of this document.

For a compat mode change, however, the list of injected stylesheets doesn't
change, so the list of matching stylesheets (the cache) should not change, but
their contents need to be reparsed. If the cache is empty, there are no
stylesheets to reparse.

A question is if injected stylesheets really should or need follow the
compatMode of the document into which they are inserted.

+        invalidateInjectedStyleSheetCache();
+}
+
 void StyleEngine::addAuthorSheet(PassRefPtrWillBeRawPtr<StyleSheetContents> authorSheet)
 {
     m_authorStyleSheets.append(CSSStyleSheet::create(authorSheet, m_document));