Revert "CSS: Drop the quirks-mode exception for CSS MIME types."

Source/core/css/StyleSheetContents.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2006, 2007, 2012 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
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 269 matching lines @@
     if (prefix == starAtom)
         return starAtom; // We'll match any namespace.
     return m_namespaces.get(prefix);
 }
 
 void StyleSheetContents::parseAuthorStyleSheet(const CSSStyleSheetResource* cachedStyleSheet, const SecurityOrigin* securityOrigin)
 {
     TRACE_EVENT0("blink", "StyleSheetContents::parseAuthorStyleSheet");
     TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("devtools.timeline"), "ParseAuthorStyleSheet", "data", InspectorParseAuthorStyleSheetEvent::data(cachedStyleSheet));
 
+    bool quirksMode = isQuirksModeBehavior(m_parserContext.mode());
+
+    bool enforceMIMEType = !quirksMode;
     bool hasValidMIMEType = false;
-    String sheetText = cachedStyleSheet->sheetText(&hasValidMIMEType);
+    String sheetText = cachedStyleSheet->sheetText(enforceMIMEType, &hasValidMIMEType);
 
     CSSParserContext context(parserContext(), UseCounter::getFrom(this));
     CSSParser::parseSheet(context, this, sheetText, TextPosition::minimumPosition(), 0, true);
 
     // If we're loading a stylesheet cross-origin, and the MIME type is not standard, require the CSS
     // to at least start with a syntactically valid CSS rule.
     // This prevents an attacker playing games by injecting CSS strings into HTML, XML, JSON, etc. etc.
     if (!hasValidMIMEType && !hasSyntacticallyValidCSSHeader()) {
         bool isCrossOriginCSS = !securityOrigin || !securityOrigin->canRequest(baseURL());
         if (isCrossOriginCSS) {
@@ skipping 347 matching lines @@
     visitor->trace(m_ownerRule);
     visitor->trace(m_importRules);
     visitor->trace(m_childRules);
     visitor->trace(m_loadingClients);
     visitor->trace(m_completedClients);
     visitor->trace(m_ruleSet);
 #endif
 }
 
 }