diff --git a/layout/style/nsStyleUtil.cpp b/layout/style/nsStyleUtil.cpp
index 6f6ceeccd65..74dac0f1ae9 100644
--- a/layout/style/nsStyleUtil.cpp
+++ b/layout/style/nsStyleUtil.cpp
@@ -81,13 +81,16 @@ void nsStyleUtil::AppendEscapedCSSString(const nsAString& aString,
 nsStyleUtil::AppendEscapedCSSIdent(const nsAString& aIdent, nsAString& aReturn)
 {
   // The relevant parts of the CSS grammar are:
-  //   ident    [-]?{nmstart}{nmchar}*
+  //   ident    ([-]?{nmstart}|[-][-]){nmchar}*
   //   nmstart  [_a-z]|{nonascii}|{escape}
   //   nmchar   [_a-z0-9-]|{nonascii}|{escape}
   //   nonascii [^\0-\177]
   //   escape   {unicode}|\\[^\n\r\f0-9a-f]
   //   unicode  \\[0-9a-f]{1,6}(\r\n|[ \n\r\t\f])?
-  // from http://www.w3.org/TR/CSS21/syndata.html#tokenization
+  // from http://www.w3.org/TR/CSS21/syndata.html#tokenization but
+  // modified for idents by
+  // http://dev.w3.org/csswg/cssom/#serialize-an-identifier and
+  // http://dev.w3.org/csswg/css-syntax/#would-start-an-identifier
 
   const char16_t* in = aIdent.BeginReading();
   const char16_t* const end = aIdent.EndReading();
@@ -97,7 +100,13 @@ nsStyleUtil::AppendEscapedCSSIdent(const nsAString& aIdent, nsAString& aReturn)
 
   // A leading dash does not need to be escaped as long as it is not the
   // *only* character in the identifier.
-  if (in + 1 != end && *in == '-') {
+  if (*in == '-') {
+    if (in + 1 == end) {
+      aReturn.Append(char16_t('\\'));
+      aReturn.Append(char16_t('-'));
+      return true;
+    }
+
     aReturn.Append(char16_t('-'));
     ++in;
   }
@@ -105,16 +114,8 @@ nsStyleUtil::AppendEscapedCSSIdent(const nsAString& aIdent, nsAString& aReturn)
   // Escape a digit at the start (including after a dash),
   // numerically.  If we didn't escape it numerically, it would get
   // interpreted as a numeric escape for the wrong character.
-  // A second dash immediately after a leading dash must also be
-  // escaped, but this may be done symbolically.
-  if (in != end && (*in == '-' ||
-                    ('0' <= *in && *in <= '9'))) {
-    if (*in == '-') {
-      aReturn.Append(char16_t('\\'));
-      aReturn.Append(char16_t('-'));
-    } else {
-      aReturn.AppendPrintf("\\%hX ", *in);
-    }
+  if (in != end && ('0' <= *in && *in <= '9')) {
+    aReturn.AppendPrintf("\\%hX ", *in);
     ++in;
   }
 
diff --git a/layout/style/test/test_css_escape_api.html b/layout/style/test/test_css_escape_api.html
index d262df09500..852c00a3b91 100644
--- a/layout/style/test/test_css_escape_api.html
+++ b/layout/style/test/test_css_escape_api.html
@@ -66,7 +66,7 @@ is(CSS.escape('-7a'), '-\\37 a', "escapingFailed Char: -7a");
 is(CSS.escape('-8a'), '-\\38 a', "escapingFailed Char: -8a");
 is(CSS.escape('-9a'), '-\\39 a', "escapingFailed Char: -9a");
 
-is(CSS.escape('--a'), '-\\-a', "escapingFailed Char: --a");
+is(CSS.escape('--a'), '--a', 'Should not need to escape leading "--"');
 
 is(CSS.escape('\x80\x2D\x5F\xA9'), '\\80 \x2D\x5F\xA9', "escapingFailed Char: \\x80\\x2D\\x5F\\xA9");
 is(CSS.escape('\xA0\xA1\xA2'), '\xA0\xA1\xA2', "escapingFailed Char: \\xA0\\xA1\\xA2");
diff --git a/layout/style/test/test_parser_diagnostics_unprintables.html b/layout/style/test/test_parser_diagnostics_unprintables.html
index e532c2f49bf..e59989e94c0 100644
--- a/layout/style/test/test_parser_diagnostics_unprintables.html
+++ b/layout/style/test/test_parser_diagnostics_unprintables.html
@@ -74,7 +74,7 @@ const substitutions = [
   { t: "\\37 ",  i: "\\37 ",  s: "7"  },
   { t: "\\38 ",  i: "\\38 ",  s: "8"  },
   { t: "\\39 ",  i: "\\39 ",  s: "9"  },
-  { t: "-\\-",   i: "-\\-",   s: "--" },
+  { t: "-\\-",   i: "--",     s: "--" },
   { t: "-\\30 ", i: "-\\30 ", s: "-0" },
   { t: "-\\31 ", i: "-\\31 ", s: "-1" },
   { t: "-\\32 ", i: "-\\32 ", s: "-2" },