diff --git a/dom/webidl/CSS.webidl b/dom/webidl/CSS.webidl
index 9965554a881..b5cdab2d185 100644
--- a/dom/webidl/CSS.webidl
+++ b/dom/webidl/CSS.webidl
@@ -20,6 +20,5 @@ interface CSS {
 
 // http://dev.w3.org/csswg/cssom/#the-css.escape%28%29-method
 partial interface CSS {
-  [Throws]
   static DOMString escape(DOMString ident);
 };
diff --git a/layout/style/CSS.cpp b/layout/style/CSS.cpp
index a0ec3a2f04d..55a426de394 100644
--- a/layout/style/CSS.cpp
+++ b/layout/style/CSS.cpp
@@ -85,14 +85,9 @@ CSS::Supports(const GlobalObject& aGlobal,
 /* static */ void
 CSS::Escape(const GlobalObject& aGlobal,
             const nsAString& aIdent,
-            nsAString& aReturn,
-            ErrorResult& aRv)
+            nsAString& aReturn)
 {
-  bool success = nsStyleUtil::AppendEscapedCSSIdent(aIdent, aReturn);
-
-  if (!success) {
-    aRv.Throw(NS_ERROR_DOM_INVALID_CHARACTER_ERR);
-  }
+  nsStyleUtil::AppendEscapedCSSIdent(aIdent, aReturn);
 }
 
 } // namespace dom
diff --git a/layout/style/CSS.h b/layout/style/CSS.h
index adf583d9509..ce3e38da41e 100644
--- a/layout/style/CSS.h
+++ b/layout/style/CSS.h
@@ -35,8 +35,7 @@ public:
 
   static void Escape(const GlobalObject& aGlobal,
                      const nsAString& aIdent,
-                     nsAString& aReturn,
-                     ErrorResult& aRv);
+                     nsAString& aReturn);
 };
 
 } // namespace dom
diff --git a/layout/style/nsStyleUtil.cpp b/layout/style/nsStyleUtil.cpp
index d1f9b4461a5..e6cbdd8a93f 100644
--- a/layout/style/nsStyleUtil.cpp
+++ b/layout/style/nsStyleUtil.cpp
@@ -79,7 +79,7 @@ void nsStyleUtil::AppendEscapedCSSString(const nsAString& aString,
   aReturn.Append(quoteChar);
 }
 
-/* static */ bool
+/* static */ void
 nsStyleUtil::AppendEscapedCSSIdent(const nsAString& aIdent, nsAString& aReturn)
 {
   // The relevant parts of the CSS grammar are:
@@ -98,7 +98,7 @@ nsStyleUtil::AppendEscapedCSSIdent(const nsAString& aIdent, nsAString& aReturn)
   const char16_t* const end = aIdent.EndReading();
 
   if (in == end)
-    return true;
+    return;
 
   // A leading dash does not need to be escaped as long as it is not the
   // *only* character in the identifier.
@@ -106,7 +106,7 @@ nsStyleUtil::AppendEscapedCSSIdent(const nsAString& aIdent, nsAString& aReturn)
     if (in + 1 == end) {
       aReturn.Append(char16_t('\\'));
       aReturn.Append(char16_t('-'));
-      return true;
+      return;
     }
 
     aReturn.Append(char16_t('-'));
@@ -124,9 +124,8 @@ nsStyleUtil::AppendEscapedCSSIdent(const nsAString& aIdent, nsAString& aReturn)
   for (; in != end; ++in) {
     char16_t ch = *in;
     if (ch == 0x00) {
-      return false;
-    }
-    if (ch < 0x20 || (0x7F <= ch && ch < 0xA0)) {
+      aReturn.Append(char16_t(0xFFFD));
+    } else if (ch < 0x20 || (0x7F <= ch && ch < 0xA0)) {
       // Escape U+0000 through U+001F and U+007F through U+009F numerically.
       aReturn.AppendPrintf("\\%hx ", *in);
     } else {
@@ -142,7 +141,6 @@ nsStyleUtil::AppendEscapedCSSIdent(const nsAString& aIdent, nsAString& aReturn)
       aReturn.Append(ch);
     }
   }
-  return true;
 }
 
 // unquoted family names must be a sequence of idents
diff --git a/layout/style/nsStyleUtil.h b/layout/style/nsStyleUtil.h
index 196563a554f..2fb098f403a 100644
--- a/layout/style/nsStyleUtil.h
+++ b/layout/style/nsStyleUtil.h
@@ -39,10 +39,9 @@ public:
 
   // Append the identifier given by |aIdent| to |aResult|, with
   // appropriate escaping so that it can be reparsed to the same
-  // identifier.
-  // Returns false if |aIdent| contains U+0000
-  // Returns true for all other cases
-  static bool AppendEscapedCSSIdent(const nsAString& aIdent,
+  // identifier.  An exception is if aIdent contains U+0000, which
+  // will be escaped as U+FFFD and then reparsed back to U+FFFD.
+  static void AppendEscapedCSSIdent(const nsAString& aIdent,
                                     nsAString& aResult);
 
   static void
diff --git a/layout/style/test/test_css_escape_api.html b/layout/style/test/test_css_escape_api.html
index 3154f4dca5a..00ec240c70d 100644
--- a/layout/style/test/test_css_escape_api.html
+++ b/layout/style/test/test_css_escape_api.html
@@ -21,11 +21,18 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=955860
 // Tests taken from:
 // https://github.com/mathiasbynens/CSS.escape/blob/master/tests/tests.js
 
-SimpleTest.doesThrow(() => CSS.escape('\0'), "InvalidCharacterError Character :\\0");
-SimpleTest.doesThrow(() => CSS.escape('a\0'), "InvalidCharacterError Character : a\\0");
-SimpleTest.doesThrow(() => CSS.escape('\0b'), "InvalidCharacterError Character : \\0b");
-SimpleTest.doesThrow(() => CSS.escape('a\0b'), "InvalidCharacterError Character : a\\0b");
 SimpleTest.doesThrow(() => CSS.escape(), 'undefined');
+
+is(CSS.escape('\0'), '\uFFFD', "escaping for 0 char (1)");
+is(CSS.escape('a\0'), 'a\uFFFD', "escaping for 0 char (2)");
+is(CSS.escape('\0b'), '\uFFFDb', "escaping for 0 char (3)");
+is(CSS.escape('a\0b'), 'a\uFFFDb', "escaping for 0 char (4)");
+
+is(CSS.escape('\uFFFD'), '\uFFFD', "escaping for replacement char (1)");
+is(CSS.escape('a\uFFFD'), 'a\uFFFD', "escaping replacement char (2)");
+is(CSS.escape('\uFFFDb'), '\uFFFDb', "escaping replacement char (3)");
+is(CSS.escape('a\uFFFDb'), 'a\uFFFDb', "escaping replacement char (4)");
+
 is(CSS.escape(true), 'true', "escapingFailed Character : true(bool)");
 is(CSS.escape(false), 'false', "escapingFailed Character : false(bool)");
 is(CSS.escape(null), 'null', "escapingFailed Character : null");