CSS escape doesn't escape leading hyphen.

Source/core/css/CSSMarkup.cpp

 /*
  * Copyright (C) 2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2012 Adobe Systems Incorporated. All rights reserved.
  * Copyright (C) 2012 Intel Corporation. All rights reserved.
  *
@@ skipping 201 matching lines @@
             // Check for lone surrogate which characterStartingAt does not return.
             c = identifier[index];
             if (c == 0)
                 return false;
         }
 
         index += U16_LENGTH(c);
 
         if (c <= 0x1f || c == 0x7f || (0x30 <= c && c <= 0x39 && (isFirst || (isSecond && isFirstCharHyphen))))
             serializeCharacterAsCodePoint(c, appendTo);
+        else if (c == 0x2d && isFirst && index == identifier.length())
+            serializeCharacter(c, appendTo);
         else if (0x80 <= c || c == 0x2d || c == 0x5f || (0x30 <= c && c <= 0x39) || (0x41 <= c && c <= 0x5a) || (0x61 <= c && c <= 0x7a))
             appendTo.append(c);
         else
             serializeCharacter(c, appendTo);
 
         if (isFirst) {
             isFirst = false;
             isSecond = true;
             isFirstCharHyphen = (c == 0x2d);
         } else if (isSecond) {
@@ skipping 23 matching lines @@
 }
 
 String serializeString(const String& string)
 {
     StringBuilder builder;
     serializeString(string, builder);
     return builder.toString();
 }
 
 } // namespace blink