Backed out changeset f7e1426249f2 (bug 949651) for mochitest-5 failures.

layout/style/StyleRule.cpp

@@ -677,10 +677,9 @@ nsCSSSelector::AppendToStringWithoutCombinatorsOrNegations
         // XXXldb Why?
         aString.Append(char16_t('*'));
       }
-      // While our atoms use one colon, most pseudo-elements require two
-      // colons (those not in CSS level 2) and all pseudo-elements allow
-      // two colons. So serialize to the non-deprecated two colon syntax.
-      aString.Append(char16_t(':'));
+      if (!nsCSSPseudoElements::IsCSS2PseudoElement(mLowercaseTag)) {
+        aString.Append(char16_t(':'));
+      }
       // This should not be escaped since (a) the pseudo-element string
       // has a ":" that can't be escaped and (b) all pseudo-elements at
       // this point are known, and therefore we know they don't need

layout/style/test/test_selectors.html

@@ -867,13 +867,9 @@ function run() {
                          emptyset, classset(["a", "b", "nomatch"]));
 
     // Test serialization of pseudo-elements.
-    should_serialize_to("p::first-letter", "p::first-letter");
-    should_serialize_to("p:first-letter", "p::first-letter");
-    should_serialize_to("div>p:first-letter", "div > p::first-letter");
-    should_serialize_to("span +div:first-line", "span + div::first-line");
-
-    // Test serialization of non CSS2 pseudo-element.
-    should_serialize_to("input::-moz-placeholder", "input::-moz-placeholder");
+    should_serialize_to("p:first-letter", "p:first-letter");
+    should_serialize_to("div>p:first-letter", "div > p:first-letter");
+    should_serialize_to("span +div:first-line", "span + div:first-line");
 
     // Test default namespaces, including inside :not().
     var html_default_ns = "@namespace url(http://www.w3.org/1999/xhtml);";