Don't try to set the empty string as the preferred set.

third_party/WebKit/Source/core/dom/StyleEngineTest.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/dom/StyleEngine.h"
 
 #include <memory>
 #include "bindings/core/v8/V8BindingForCore.h"
 #include "core/css/CSSRuleList.h"
 #include "core/css/CSSStyleRule.h"
@@ skipping 539 matching lines @@
       "  t1 { color: green }"
       "</style>"
       "<div id='t1'></div>"
       "<span></span>");
 
   GetStyleEngine().UpdateActiveStyle();
   EXPECT_FALSE(GetDocument().ChildNeedsStyleInvalidation());
   EXPECT_FALSE(GetDocument().NeedsStyleInvalidation());
 }
 
+TEST_F(StyleEngineTest, EmptyHttpEquivDefaultStyle) {
+  GetDocument().body()->setInnerHTML(
+      "<style>div { color:pink }</style><div id=container></div>");
+  GetDocument().View()->UpdateAllLifecyclePhases();
+
+  EXPECT_FALSE(GetStyleEngine().NeedsActiveStyleUpdate());
+
+  Element* container = GetDocument().getElementById("container");
+  ASSERT_TRUE(container);
+  container->setInnerHTML("<meta http-equiv='default-style' content=''>");
+  EXPECT_FALSE(GetStyleEngine().NeedsActiveStyleUpdate());
+
+  container->setInnerHTML(
+      "<meta http-equiv='default-style' content='preferred'>");
+  EXPECT_TRUE(GetStyleEngine().NeedsActiveStyleUpdate());
+}
+
 }  // namespace blink