--- a/layout/style/StyleRule.cpp
+++ b/layout/style/StyleRule.cpp
@@ -672,19 +672,20 @@ nsCSSSelector::AppendToStringWithoutComb
     nsAutoString tag;
     (isPseudoElement ? mLowercaseTag : mCasedTag)->ToString(tag);
     if (isPseudoElement) {
       if (!mNext) {
         // Lone pseudo-element selector -- toss in a wildcard type selector
         // XXXldb Why?
         aString.Append(char16_t('*'));
       }
-      if (!nsCSSPseudoElements::IsCSS2PseudoElement(mLowercaseTag)) {
-        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(':'));
       // 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
       // escaping.
       aString.Append(tag);
     } else {
       nsStyleUtil::AppendEscapedCSSIdent(tag, aString);
     }

--- a/layout/style/test/test_selectors.html
+++ b/layout/style/test/test_selectors.html
@@ -862,19 +862,23 @@ function run() {
     test_selector_in_html(".a > .b ~ .nomatch", '<div class="a"><div><div class="b"></div><div class="nomatch"></div></div><div class="nomatch"></div></div>',
                          emptyset, classset(["a", "b", "nomatch"]));
     test_selector_in_html(".a > .b ~ .nomatch", '<div class="a"><div class="b"></div><div><div class="nomatch"></div></div><div></div></div>',
                          emptyset, classset(["a", "b", "nomatch"]));
     test_selector_in_html(".a > .b ~ .nomatch", '<div class="a"><div class="b"></div></div><div class="nomatch"></div>',
                          emptyset, classset(["a", "b", "nomatch"]));
 
     // Test serialization of pseudo-elements.
-    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");
+    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");
 
     // Test default namespaces, including inside :not().
     var html_default_ns = "@namespace url(http://www.w3.org/1999/xhtml);";
     var html_ns = "@namespace html url(http://www.w3.org/1999/xhtml);";
     var xul_default_ns = "@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);";
     var single_a = "<a id='a' href='data:text/plain,this_better_be_unvisited'></a>";
     var set_single = idset(['a']);
     var empty_set = idset([]);