--- a/devtools/client/styleinspector/rule-view.js
+++ b/devtools/client/styleinspector/rule-view.js
@@ -8,18 +8,17 @@
 
 const {Cc, Ci, Cu} = require("chrome");
 const promise = require("promise");
 const {setTimeout, clearTimeout} =
       Cu.import("resource://gre/modules/Timer.jsm", {});
 const {CssLogic} = require("devtools/shared/styleinspector/css-logic");
 const {InplaceEditor, editableField, editableItem} =
       require("devtools/client/shared/inplace-editor");
-const {ELEMENT_STYLE, PSEUDO_ELEMENTS} =
-      require("devtools/server/actors/styles");
+const {ELEMENT_STYLE} = require("devtools/server/actors/styles");
 const {OutputParser} = require("devtools/shared/output-parser");
 const {PrefObserver, PREF_ORIG_SOURCES} = require("devtools/client/styleeditor/utils");
 const {
   createChild,
   appendText,
   advanceValidate,
   blurOnMultipleProperties,
   promiseWarn,
@@ -4054,8 +4053,12 @@ XPCOMUtils.defineLazyGetter(this, "_stri
 
 XPCOMUtils.defineLazyGetter(this, "domUtils", function() {
   return Cc["@mozilla.org/inspector/dom-utils;1"].getService(Ci.inIDOMUtils);
 });
 
 loader.lazyGetter(this, "AutocompletePopup", function() {
   return require("devtools/client/shared/autocomplete-popup").AutocompletePopup;
 });
+
+loader.lazyGetter(this, "PSEUDO_ELEMENTS", () => {
+  return domUtils.getCSSPseudoElementNames();
+});

--- a/devtools/client/styleinspector/test/browser_ruleview_user-agent-styles.js
+++ b/devtools/client/styleinspector/test/browser_ruleview_user-agent-styles.js
@@ -155,16 +155,24 @@ function* compareAppliedStylesWithUI(ins
 
   let entries = yield inspector.pageStyle.getApplied(
     inspector.selection.nodeFront, {
     inherited: true,
     matchedSelectors: true,
     filter: filter
   });
 
+  // We may see multiple entries that map to a given rule; filter the
+  // duplicates here to match what the UI does.
+  let entryMap = new Map();
+  for (let entry of entries) {
+    entryMap.set(entry.rule, entry);
+  }
+  entries = [...entryMap.values()];
+
   let elementStyle = view._elementStyle;
   is(elementStyle.rules.length, entries.length,
     "Should have correct number of rules (" + entries.length + ")");
 
   entries = entries.sort((a, b) => {
     return (a.pseudoElement || "z") > (b.pseudoElement || "z");
   });
 

--- a/devtools/server/actors/styles.js
+++ b/devtools/server/actors/styles.js
@@ -7,17 +7,16 @@
 
 const {Cc, Ci, Cu} = require("chrome");
 const promise = require("promise");
 const protocol = require("devtools/server/protocol");
 const {Arg, Option, method, RetVal, types} = protocol;
 const events = require("sdk/event/core");
 const {Class} = require("sdk/core/heritage");
 const {LongStringActor} = require("devtools/server/actors/string");
-const {PSEUDO_ELEMENT_SET} = require("devtools/shared/styleinspector/css-logic");
 
 // This will also add the "stylesheet" actor type for protocol.js to recognize
 const {UPDATE_PRESERVING_RULES, UPDATE_GENERAL} =
       require("devtools/server/actors/stylesheets");
 
 loader.lazyGetter(this, "CssLogic", () => {
   return require("devtools/shared/styleinspector/css-logic").CssLogic;
 });
@@ -30,34 +29,22 @@ loader.lazyGetter(this, "RuleRewriter", 
 });
 
 // The PageStyle actor flattens the DOM CSS objects a little bit, merging
 // Rules and their Styles into one actor.  For elements (which have a style
 // but no associated rule) we fake a rule with the following style id.
 const ELEMENT_STYLE = 100;
 exports.ELEMENT_STYLE = ELEMENT_STYLE;
 
-// Not included since these are uneditable by the user.
-// See https://hg.mozilla.org/mozilla-central/file/696a4ad5d011/layout/style/nsCSSPseudoElementList.h#l74
-PSEUDO_ELEMENT_SET.delete(":-moz-meter-bar");
-PSEUDO_ELEMENT_SET.delete(":-moz-list-bullet");
-PSEUDO_ELEMENT_SET.delete(":-moz-list-number");
-PSEUDO_ELEMENT_SET.delete(":-moz-focus-inner");
-PSEUDO_ELEMENT_SET.delete(":-moz-focus-outer");
-PSEUDO_ELEMENT_SET.delete(":-moz-math-anonymous");
-PSEUDO_ELEMENT_SET.delete(":-moz-math-stretchy");
-
-const PSEUDO_ELEMENTS = Array.from(PSEUDO_ELEMENT_SET);
-
-exports.PSEUDO_ELEMENTS = PSEUDO_ELEMENTS;
-
 // When gathering rules to read for pseudo elements, we will skip
 // :before and :after, which are handled as a special case.
-const PSEUDO_ELEMENTS_TO_READ = PSEUDO_ELEMENTS.filter(pseudo => {
-  return pseudo !== ":before" && pseudo !== ":after";
+loader.lazyGetter(this, "PSEUDO_ELEMENTS_TO_READ", () => {
+  return DOMUtils.getCSSPseudoElementNames().filter(pseudo => {
+    return pseudo !== ":before" && pseudo !== ":after";
+  });
 });
 
 const XHTML_NS = "http://www.w3.org/1999/xhtml";
 const FONT_PREVIEW_TEXT = "Abc";
 const FONT_PREVIEW_FONT_SIZE = 40;
 const FONT_PREVIEW_FILLSTYLE = "black";
 const NORMAL_FONT_WEIGHT = 400;
 const BOLD_FONT_WEIGHT = 700;

--- a/devtools/server/tests/mochitest/test_styles-applied.html
+++ b/devtools/server/tests/mochitest/test_styles-applied.html
@@ -80,17 +80,17 @@ addTest(function inheritedSystemStyles()
     // If our system stylesheets are prone to churn, this might be a fragile
     // test.  If you're here because of that I apologize, file a bug
     // and we can find a different way to test.
 
     ok(!applied[1].inherited, "Entry 1 should not be inherited");
     ok(!applied[1].rule.parentStyleSheet.system, "Entry 1 should be a system style");
     is(applied[1].rule.type, 1, "Entry 1 should be a rule style");
 
-    is(applied.length, 8, "Should have 8 rules.");
+    is(applied.length, 11, "Should have 11 rules.");
   }).then(runNextTest));
 });
 
 addTest(function noInheritedStyles() {
   promiseDone(gWalker.querySelector(gWalker.rootNode, "#test-node").then(node => {
     return gStyles.getApplied(node, { inherited: false, filter: "user" });
   }).then(applied => {
     ok(!applied[0].inherited, "Entry 0 should be uninherited");

--- a/devtools/shared/styleinspector/css-logic.js
+++ b/devtools/shared/styleinspector/css-logic.js
@@ -38,46 +38,16 @@
  * @constructor
  */
 
 const { Cc, Ci, Cu } = require("chrome");
 const Services = require("Services");
 const DevToolsUtils = require("devtools/shared/DevToolsUtils");
 const { getRootBindingParent } = require("devtools/shared/layout/utils");
 
-var pseudos = new Set([
-  ":after",
-  ":before",
-  ":first-letter",
-  ":first-line",
-  ":selection",
-  ":-moz-color-swatch",
-  ":-moz-focus-inner",
-  ":-moz-focus-outer",
-  ":-moz-list-bullet",
-  ":-moz-list-number",
-  ":-moz-math-anonymous",
-  ":-moz-math-stretchy",
-  ":-moz-meter-bar",
-  ":-moz-number-spin-box",
-  ":-moz-number-spin-down",
-  ":-moz-number-spin-up",
-  ":-moz-number-text",
-  ":-moz-number-wrapper",
-  ":-moz-placeholder",
-  ":-moz-progress-bar",
-  ":-moz-range-progress",
-  ":-moz-range-thumb",
-  ":-moz-range-track",
-  ":-moz-selection"
-]);
-
-const PSEUDO_ELEMENT_SET = pseudos;
-exports.PSEUDO_ELEMENT_SET = PSEUDO_ELEMENT_SET;
-
 // This should be ok because none of the functions that use this should be used
 // on the worker thread, where Cu is not available.
 if (Cu) {
   Cu.importGlobalProperties(["CSS"]);
 }
 
 function CssLogic()
 {
@@ -1674,28 +1644,16 @@ CssSelector.prototype = {
    * stylesheet.
    */
   get ruleLine()
   {
     return this.cssRule.line;
   },
 
   /**
-   * Retrieve the pseudo-elements that we support. This list should match the
-   * elements specified in layout/style/nsCSSPseudoElementList.h
-   */
-  get pseudoElements()
-  {
-    if (!CssSelector._pseudoElements) {
-      CssSelector._pseudoElements = PSEUDO_ELEMENT_SET;
-    }
-    return CssSelector._pseudoElements;
-  },
-
-  /**
    * Retrieve specificity information for the current selector.
    *
    * @see http://www.w3.org/TR/css3-selectors/#specificity
    * @see http://www.w3.org/TR/CSS2/selector.html
    *
    * @return {Number} The selector's specificity.
    */
   get specificity()

--- a/layout/inspector/inDOMUtils.cpp
+++ b/layout/inspector/inDOMUtils.cpp
@@ -1187,16 +1187,39 @@ GetStatesForPseudoClass(const nsAString&
     return EventStates();
   }
   // Our array above is long enough that indexing into it with
   // NotPseudoClass is ok.
   return sPseudoClassStates[nsCSSPseudoClasses::GetPseudoType(atom)];
 }
 
 NS_IMETHODIMP
+inDOMUtils::GetCSSPseudoElementNames(uint32_t* aLength, char16_t*** aNames)
+{
+  nsTArray<nsIAtom*> array;
+
+  for (int i = 0; i < nsCSSPseudoElements::ePseudo_PseudoElementCount; ++i) {
+    nsCSSPseudoElements::Type type = static_cast<nsCSSPseudoElements::Type>(i);
+    if (!nsCSSPseudoElements::PseudoElementIsUASheetOnly(type)) {
+      nsIAtom* atom = nsCSSPseudoElements::GetPseudoAtom(type);
+      array.AppendElement(atom);
+    }
+  }
+
+  *aLength = array.Length();
+  char16_t** ret =
+    static_cast<char16_t**>(moz_xmalloc(*aLength * sizeof(char16_t*)));
+  for (uint32_t i = 0; i < *aLength; ++i) {
+    ret[i] = ToNewUnicode(nsDependentAtomString(array[i]));
+  }
+  *aNames = ret;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
 inDOMUtils::AddPseudoClassLock(nsIDOMElement *aElement,
                                const nsAString &aPseudoClass)
 {
   EventStates state = GetStatesForPseudoClass(aPseudoClass);
   if (state.IsEmpty()) {
     return NS_OK;
   }
 

--- a/layout/inspector/inIDOMUtils.idl
+++ b/layout/inspector/inIDOMUtils.idl
@@ -12,17 +12,17 @@ interface nsIDOMDocument;
 interface nsIDOMCSSRule;
 interface nsIDOMCSSStyleRule;
 interface nsIDOMNode;
 interface nsIDOMNodeList;
 interface nsIDOMFontFaceList;
 interface nsIDOMRange;
 interface nsIDOMCSSStyleSheet;
 
-[scriptable, uuid(d67c0463-592e-4d7c-b67e-923ee3f6c643)]
+[scriptable, uuid(ec3dc3d5-41d1-4d08-ace5-7e944de6614d)]
 interface inIDOMUtils : nsISupports
 {
   // CSS utilities
   void getAllStyleSheets (in nsIDOMDocument aDoc,
                           [optional] out unsigned long aLength,
                           [array, size_is (aLength), retval] out nsISupports aSheets);
   nsISupportsArray getCSSStyleRules(in nsIDOMElement aElement, [optional] in DOMString aPseudo);
 
@@ -157,16 +157,27 @@ interface inIDOMUtils : nsISupports
   nsIArray getBindingURLs(in nsIDOMElement aElement);
 
   // content state utilities
   unsigned long long getContentState(in nsIDOMElement aElement);
   void setContentState(in nsIDOMElement aElement, in unsigned long long aState);
 
   nsIDOMFontFaceList getUsedFontFaces(in nsIDOMRange aRange);
 
+  /**
+   * Get the names of all the supported pseudo-elements.
+   * Pseudo-elements which are only accepted in UA style sheets are
+   * not included.
+   *
+   * @param {unsigned long} aCount the number of items returned
+   * @param {wstring[]} aNames the names
+   */
+  void getCSSPseudoElementNames([optional] out unsigned long aCount,
+                                [retval, array, size_is(aCount)] out wstring aNames);
+
   // pseudo-class style locking methods. aPseudoClass must be a valid pseudo-class
   // selector string, e.g. ":hover". ":-moz-any-link" and non-event-state
   // pseudo-classes are ignored.
   void addPseudoClassLock(in nsIDOMElement aElement, in DOMString aPseudoClass);
   void removePseudoClassLock(in nsIDOMElement aElement, in DOMString aPseudoClass);
   bool hasPseudoClassLock(in nsIDOMElement aElement, in DOMString aPseudoClass);
   void clearPseudoClassLocks(in nsIDOMElement aElement);
 

--- a/layout/inspector/tests/mochitest.ini
+++ b/layout/inspector/tests/mochitest.ini
@@ -14,15 +14,16 @@ support-files =
 [test_bug609549.xhtml]
 [test_bug806192.html]
 [test_bug856317.html]
 [test_bug877690.html]
 [test_bug1006595.html]
 [test_color_to_rgba.html]
 [test_css_property_is_shorthand.html]
 [test_css_property_is_valid.html]
+[test_getCSSPseudoElementNames.html]
 [test_getRelativeRuleLine.html]
 [test_get_all_style_sheets.html]
 [test_is_valid_css_color.html]
 [test_isinheritableproperty.html]
 [test_parseStyleSheet.html]
 [test_parseStyleSheetImport.html]
 [test_selectormatcheselement.html]

new file mode 100644
--- /dev/null
+++ b/layout/inspector/tests/test_getCSSPseudoElementNames.html
@@ -0,0 +1,57 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>Test inDOMUtils::getCSSPseudoElementNames</title>
+  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+  <script type="application/javascript;version=1.8">
+  let utils = SpecialPowers.Cc["@mozilla.org/inspector/dom-utils;1"]
+                           .getService(SpecialPowers.Ci.inIDOMUtils);
+
+  let expected = new Set([
+    ":after",
+    ":before",
+    ":first-letter",
+    ":first-line",
+    ":-moz-color-swatch",
+    ":-moz-focus-inner",
+    ":-moz-focus-outer",
+    ":-moz-list-bullet",
+    ":-moz-list-number",
+    ":-moz-math-anonymous",
+    ":-moz-meter-bar",
+    ":-moz-placeholder",
+    ":-moz-progress-bar",
+    ":-moz-range-progress",
+    ":-moz-range-thumb",
+    ":-moz-range-track",
+    ":-moz-selection",
+  ]);
+
+  let names = utils.getCSSPseudoElementNames();
+  for (let name of names) {
+    ok(expected.has(name), name + " is included");
+    expected.delete(name);
+  }
+
+  if (expected.size > 0) {
+    todo_is(expected.size, 0,
+            "ideally all pseudo-element names would be listed in this test");
+    for (let extra of expected) {
+      info("extra element: " + extra);
+    }
+  }
+
+  </script>
+</head>
+<body>
+<h1>Test inDOMUtils::getCSSPseudoElementNames</h1>
+<p id="display"></p>
+<div id="content" style="display: none">
+
+</div>
+<pre id="test">
+</pre>
+</body>
+</html>