--- a/dom/html/HTMLDetailsElement.cpp
+++ b/dom/html/HTMLDetailsElement.cpp
@@ -1,49 +1,22 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/dom/HTMLDetailsElement.h"
 
 #include "mozilla/dom/HTMLDetailsElementBinding.h"
-#include "mozilla/dom/HTMLUnknownElement.h"
-#include "mozilla/Preferences.h"
 
-// Expand NS_IMPL_NS_NEW_HTML_ELEMENT(Details) to add pref check.
-nsGenericHTMLElement*
-NS_NewHTMLDetailsElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
-                         mozilla::dom::FromParser aFromParser)
-{
-  if (!mozilla::dom::HTMLDetailsElement::IsDetailsEnabled()) {
-    return new mozilla::dom::HTMLUnknownElement(aNodeInfo);
-  }
-
-  return new mozilla::dom::HTMLDetailsElement(aNodeInfo);
-}
+NS_IMPL_NS_NEW_HTML_ELEMENT(Details)
 
 namespace mozilla {
 namespace dom {
 
-/* static */ bool
-HTMLDetailsElement::IsDetailsEnabled()
-{
-  static bool isDetailsEnabled = false;
-  static bool added = false;
-
-  if (!added) {
-    Preferences::AddBoolVarCache(&isDetailsEnabled,
-                                 "dom.details_element.enabled");
-    added = true;
-  }
-
-  return isDetailsEnabled;
-}
-
 HTMLDetailsElement::~HTMLDetailsElement()
 {
 }
 
 NS_IMPL_ELEMENT_CLONE(HTMLDetailsElement)
 
 nsIContent*
 HTMLDetailsElement::GetFirstSummary() const

--- a/dom/html/HTMLDetailsElement.h
+++ b/dom/html/HTMLDetailsElement.h
@@ -18,18 +18,16 @@ namespace dom {
 // controls. Please see the spec for more information.
 // https://html.spec.whatwg.org/multipage/forms.html#the-details-element
 //
 class HTMLDetailsElement final : public nsGenericHTMLElement
 {
 public:
   using NodeInfo = mozilla::dom::NodeInfo;
 
-  static bool IsDetailsEnabled();
-
   explicit HTMLDetailsElement(already_AddRefed<NodeInfo>& aNodeInfo)
     : nsGenericHTMLElement(aNodeInfo)
   {
   }
 
   NS_IMPL_FROMCONTENT_HTML_WITH_TAG(HTMLDetailsElement, details)
 
   nsIContent* GetFirstSummary() const;

--- a/dom/html/HTMLSummaryElement.cpp
+++ b/dom/html/HTMLSummaryElement.cpp
@@ -9,27 +9,17 @@
 #include "mozilla/dom/HTMLElementBinding.h"
 #include "mozilla/dom/HTMLUnknownElement.h"
 #include "mozilla/EventDispatcher.h"
 #include "mozilla/MouseEvents.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/TextEvents.h"
 #include "nsFocusManager.h"
 
-// Expand NS_IMPL_NS_NEW_HTML_ELEMENT(Summary) to add pref check.
-nsGenericHTMLElement*
-NS_NewHTMLSummaryElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
-                         mozilla::dom::FromParser aFromParser)
-{
-  if (!mozilla::dom::HTMLDetailsElement::IsDetailsEnabled()) {
-    return new mozilla::dom::HTMLUnknownElement(aNodeInfo);
-  }
-
-  return new mozilla::dom::HTMLSummaryElement(aNodeInfo);
-}
+NS_IMPL_NS_NEW_HTML_ELEMENT(Summary)
 
 namespace mozilla {
 namespace dom {
 
 HTMLSummaryElement::~HTMLSummaryElement()
 {
 }
 

--- a/dom/webidl/EventHandler.webidl
+++ b/dom/webidl/EventHandler.webidl
@@ -88,17 +88,16 @@ interface GlobalEventHandlers {
            attribute EventHandler onsuspend;
            attribute EventHandler ontimeupdate;
            attribute EventHandler onvolumechange;
            attribute EventHandler onwaiting;
 
            [Pref="dom.select_events.enabled"]
            attribute EventHandler onselectstart;
 
-           [Pref="dom.details_element.enabled"]
            attribute EventHandler ontoggle;
 
            // Pointer events handlers
            [Pref="dom.w3c_pointer_events.enabled"]
            attribute EventHandler onpointercancel;
            [Pref="dom.w3c_pointer_events.enabled"]
            attribute EventHandler onpointerdown;
            [Pref="dom.w3c_pointer_events.enabled"]

--- a/dom/webidl/HTMLDetailsElement.webidl
+++ b/dom/webidl/HTMLDetailsElement.webidl
@@ -6,13 +6,12 @@
  * The origin of this IDL file is
  * https://html.spec.whatwg.org/multipage/forms.html#the-details-element
  *
  * © Copyright 2004-2011 Apple Computer, Inc., Mozilla Foundation, and
  * Opera Software ASA. You are granted a license to use, reproduce
  * and create derivative works of this document.
  */
 
-[Pref="dom.details_element.enabled"]
 interface HTMLDetailsElement : HTMLElement {
   [SetterThrows]
   attribute boolean open;
 };

--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -3547,20 +3547,16 @@ nsCSSFrameConstructor::FindHTMLData(Elem
     // <legend> is only special inside fieldset, we only check the frame tree
     // parent because the content tree parent may not be a <fieldset> due to
     // display:contents, Shadow DOM, or XBL. For floated or absolutely
     // positioned legends we want to construct by display type and
     // not do special legend stuff.
     return nullptr;
   }
 
-  if (aTag == nsGkAtoms::details && !HTMLDetailsElement::IsDetailsEnabled()) {
-    return nullptr;
-  }
-
   static const FrameConstructionDataByTag sHTMLData[] = {
     SIMPLE_TAG_CHAIN(img, nsCSSFrameConstructor::FindImgData),
     SIMPLE_TAG_CHAIN(mozgeneratedcontentimage,
                      nsCSSFrameConstructor::FindImgData),
     { &nsGkAtoms::br,
       FCDATA_DECL(FCDATA_IS_LINE_PARTICIPANT | FCDATA_IS_LINE_BREAK,
                   NS_NewBRFrame) },
     SIMPLE_TAG_CREATE(wbr, NS_NewWBRFrame),
@@ -5762,17 +5758,17 @@ nsCSSFrameConstructor::AddFrameConstruct
   }
 
   // When constructing a child of a non-open <details>, create only the frame
   // for the main <summary> element, and skip other elements.  This only applies
   // to things that are not roots of native anonymous subtrees (except for
   // ::before and ::after); we always want to create "internal" anonymous
   // content.
   auto* details = HTMLDetailsElement::FromContentOrNull(parent);
-  if (details && details->IsDetailsEnabled() && !details->Open() &&
+  if (details && !details->Open() &&
       (!aContent->IsRootOfNativeAnonymousSubtree() ||
        aContent->IsGeneratedContentContainerForBefore() ||
        aContent->IsGeneratedContentContainerForAfter())) {
     auto* summary = HTMLSummaryElement::FromContentOrNull(aContent);
     if (!summary || !summary->IsMainSummary()) {
       SetAsUndisplayedContent(aState, aItems, aContent, styleContext,
                               isGeneratedContent);
       return;
@@ -5930,17 +5926,17 @@ nsCSSFrameConstructor::AddFrameConstruct
                                CSSPseudoElementType::after, aItems);
     if (canHavePageBreak && display->mBreakAfter) {
       AddPageBreakItem(aContent, aStyleContext, aItems);
     }
     return;
   }
 
   FrameConstructionItem* item = nullptr;
-  if (details && details->IsDetailsEnabled() && details->Open()) {
+  if (details && details->Open()) {
     auto* summary = HTMLSummaryElement::FromContentOrNull(aContent);
     if (summary && summary->IsMainSummary()) {
       // If details is open, the main summary needs to be rendered as if it is
       // the first child, so add the item to the front of the item list.
       item = aItems.PrependItem(data, aContent, aTag, aNameSpaceID,
                                 pendingBinding, styleContext.forget(),
                                 aSuppressWhiteSpaceOptimizations, aAnonChildren);
     }

--- a/layout/generic/crashtests/crashtests.list
+++ b/layout/generic/crashtests/crashtests.list
@@ -601,29 +601,29 @@ load 1222783.xhtml
 load 1223568-1.html
 load 1223568-2.html
 load 1224230-1.html
 pref(layout.css.grid.enabled,true) load 1225118.html
 pref(layout.css.grid.enabled,true) load 1225376.html
 pref(layout.css.grid.enabled,true) load 1225592.html
 load 1229437-1.html
 load 1229437-2.html
-pref(dom.details_element.enabled,true) load details-containing-only-text.html
-pref(dom.details_element.enabled,true) load details-display-none-summary-1.html
-pref(dom.details_element.enabled,true) load details-display-none-summary-2.html
-pref(dom.details_element.enabled,true) load details-display-none-summary-3.html
-pref(dom.details_element.enabled,true) load details-open-overflow-auto.html
-pref(dom.details_element.enabled,true) load details-open-overflow-hidden.html
-pref(dom.details_element.enabled,true) load details-three-columns.html
+load details-containing-only-text.html
+load details-display-none-summary-1.html
+load details-display-none-summary-2.html
+load details-display-none-summary-3.html
+load details-open-overflow-auto.html
+load details-open-overflow-hidden.html
+load details-three-columns.html
 load first-letter-638937-1.html
 load first-letter-638937-2.html
 load flex-nested-abspos-1.html
 pref(dom.meta-viewport.enabled,true) test-pref(font.size.inflation.emPerLine,15) asserts(0-100) load font-inflation-762332.html # bug 762332
 load outline-on-frameset.xhtml
-pref(dom.details_element.enabled,true) load summary-position-out-of-flow.html
+load summary-position-out-of-flow.html
 load text-overflow-bug666751-1.html
 load text-overflow-bug666751-2.html
 load text-overflow-bug670564.xhtml
 load text-overflow-bug671796.xhtml
 load text-overflow-bug713610.html
 load text-overflow-form-elements.html
 load text-overflow-iframe.html
 asserts-if(Android,2-4) asserts-if(!Android,4) load 1225005.html # bug 682647 and bug 448083

--- a/layout/generic/nsContainerFrame.cpp
+++ b/layout/generic/nsContainerFrame.cpp
@@ -1874,22 +1874,20 @@ nsContainerFrame::RenumberFrameAndDescen
 
   // drill down through any wrappers to the real frame
   nsIFrame* kid = GetContentInsertionFrame();
   if (!kid) {
     return false;
   }
 
   // Do not renumber list for summary elements.
-  if (HTMLDetailsElement::IsDetailsEnabled()) {
-    HTMLSummaryElement* summary =
-      HTMLSummaryElement::FromContent(kid->GetContent());
-    if (summary && summary->IsMainSummary()) {
-      return false;
-    }
+  HTMLSummaryElement* summary =
+    HTMLSummaryElement::FromContent(kid->GetContent());
+  if (summary && summary->IsMainSummary()) {
+    return false;
   }
 
   bool kidRenumberedABullet = false;
 
   // If the frame is a list-item and the frame implements our
   // block frame API then get its bullet and set the list item
   // ordinal.
   if (mozilla::StyleDisplay::ListItem == display->mDisplay) {

deleted file mode 100644
--- a/layout/reftests/details-summary/disabled-no-summary-ref.html
+++ /dev/null
@@ -1,11 +0,0 @@
-<!DOCTYPE html>
-<!-- Any copyright is dedicated to the Public Domain.
-   - http://creativecommons.org/publicdomain/zero/1.0/ -->
-
-<html>
-  <body>
-    <div>
-      <p>This is the details.</p>
-    </div>
-  </body>
-</html>

deleted file mode 100644
--- a/layout/reftests/details-summary/disabled-single-summary-ref.html
+++ /dev/null
@@ -1,12 +0,0 @@
-<!DOCTYPE html>
-<!-- Any copyright is dedicated to the Public Domain.
-   - http://creativecommons.org/publicdomain/zero/1.0/ -->
-
-<html>
-  <body>
-    <div>
-      <div>Summary</div>
-      <p>This is the details.</p>
-    </div>
-  </body>
-</html>

--- a/layout/reftests/details-summary/reftest-stylo.list
+++ b/layout/reftests/details-summary/reftest-stylo.list
@@ -1,16 +1,9 @@
 # DO NOT EDIT! This is a auto-generated temporary list for Stylo testing
-default-preferences pref(dom.details_element.enabled,true)
-
-# Disable <details> and <summary>
-pref(dom.details_element.enabled,false) == single-summary.html single-summary.html
-pref(dom.details_element.enabled,false) == open-single-summary.html open-single-summary.html
-pref(dom.details_element.enabled,false) == no-summary.html no-summary.html
-
 # Basic <summary> handling
 == multiple-summary.html multiple-summary.html
 == open-multiple-summary.html open-multiple-summary.html
 == summary-not-first-child.html summary-not-first-child.html
 == open-summary-not-first-child.html open-summary-not-first-child.html
 == open-summary-block-style.html open-summary-block-style.html
 == no-summary.html no-summary.html
 == open-no-summary.html open-no-summary.html

--- a/layout/reftests/details-summary/reftest.list
+++ b/layout/reftests/details-summary/reftest.list
@@ -1,15 +1,8 @@
-default-preferences pref(dom.details_element.enabled,true)
-
-# Disable <details> and <summary>
-pref(dom.details_element.enabled,false) == single-summary.html disabled-single-summary-ref.html
-pref(dom.details_element.enabled,false) == open-single-summary.html disabled-single-summary-ref.html
-pref(dom.details_element.enabled,false) == no-summary.html disabled-no-summary-ref.html
-
 # Basic <summary> handling
 == multiple-summary.html single-summary.html
 == open-multiple-summary.html open-multiple-summary-ref.html
 == summary-not-first-child.html single-summary.html
 == open-summary-not-first-child.html open-single-summary.html
 == open-summary-block-style.html open-summary-block-style-ref.html
 == open-summary-inline-style.html open-summary-inline-style-ref.html
 == open-summary-table-cell-style.html open-summary-table-cell-style-ref.html

--- a/layout/style/nsLayoutStylesheetCache.cpp
+++ b/layout/style/nsLayoutStylesheetCache.cpp
@@ -128,21 +128,16 @@ nsLayoutStylesheetCache::UASheet()
   }
 
   return mUASheet;
 }
 
 StyleSheet*
 nsLayoutStylesheetCache::HTMLSheet()
 {
-  if (!mHTMLSheet) {
-    LoadSheetURL("resource://gre-resources/html.css",
-                 &mHTMLSheet, eAgentSheetFeatures);
-  }
-
   return mHTMLSheet;
 }
 
 StyleSheet*
 nsLayoutStylesheetCache::MinimalXULSheet()
 {
   return mMinimalXULSheet;
 }
@@ -320,16 +315,18 @@ nsLayoutStylesheetCache::nsLayoutStylesh
   }
 
   InitFromProfile();
 
   // And make sure that we load our UA sheets.  No need to do this
   // per-profile, since they're profile-invariant.
   LoadSheetURL("resource://gre-resources/counterstyles.css",
                &mCounterStylesSheet, eAgentSheetFeatures);
+  LoadSheetURL("resource://gre-resources/html.css",
+               &mHTMLSheet, eAgentSheetFeatures);
   LoadSheetURL("chrome://global/content/minimal-xul.css",
                &mMinimalXULSheet, eAgentSheetFeatures);
   LoadSheetURL("resource://gre-resources/quirk.css",
                &mQuirkSheet, eAgentSheetFeatures);
   LoadSheetURL("resource://gre/res/svg.css",
                &mSVGSheet, eAgentSheetFeatures);
   LoadSheetURL("chrome://global/content/xul.css",
                &mXULSheet, eAgentSheetFeatures);
@@ -374,18 +371,16 @@ nsLayoutStylesheetCache::For(StyleBacken
     // For each pref that controls a CSS feature that a UA style sheet depends
     // on (such as a pref that enables a property that a UA style sheet uses),
     // register DependentPrefChanged as a callback to ensure that the relevant
     // style sheets will be re-parsed.
     // Preferences::RegisterCallback(&DependentPrefChanged,
     //                               "layout.css.example-pref.enabled");
     Preferences::RegisterCallback(&DependentPrefChanged,
                                   "layout.css.grid.enabled");
-    Preferences::RegisterCallback(&DependentPrefChanged,
-                                  "dom.details_element.enabled");
   }
 
   return cache;
 }
 
 void
 nsLayoutStylesheetCache::InitFromProfile()
 {
@@ -815,17 +810,16 @@ nsLayoutStylesheetCache::DependentPrefCh
   // then setting our cached sheet pointer to null.  This will only work for
   // sheets that are loaded lazily.
 
 #define INVALIDATE(sheet_) \
   InvalidateSheet(gStyleCache_Gecko ? &gStyleCache_Gecko->sheet_ : nullptr, \
                   gStyleCache_Servo ? &gStyleCache_Servo->sheet_ : nullptr);
 
   INVALIDATE(mUASheet);  // for layout.css.grid.enabled
-  INVALIDATE(mHTMLSheet); // for dom.details_element.enabled
 
 #undef INVALIDATE
 }
 
 /* static */ void
 nsLayoutStylesheetCache::InvalidatePreferenceSheets()
 {
   if (gStyleCache_Gecko) {

--- a/layout/style/res/html.css
+++ b/layout/style/res/html.css
@@ -770,33 +770,30 @@ video > .caption-box {
 /* datetime elements */
 
 input[type="time"] > xul|datetimebox {
   display: flex;
   -moz-binding: url("chrome://global/content/bindings/datetimebox.xml#time-input");
 }
 
 /* details & summary */
-/* Need to revert Bug 1259889 Part 2 when removing details preference. */
-@supports -moz-bool-pref("dom.details_element.enabled") {
-  details > summary:first-of-type,
-  details > summary:-moz-native-anonymous {
-    display: list-item;
-    list-style: disclosure-closed inside;
-  }
+details > summary:first-of-type,
+details > summary:-moz-native-anonymous {
+  display: list-item;
+  list-style: disclosure-closed inside;
+}
 
-  details[open] > summary:first-of-type,
-  details[open] > summary:-moz-native-anonymous {
-    list-style-type: disclosure-open;
-  }
+details[open] > summary:first-of-type,
+details[open] > summary:-moz-native-anonymous {
+  list-style-type: disclosure-open;
+}
 
-  details > summary:first-of-type > *|* {
-    /* Cancel "list-style-position: inside" inherited from summary. */
-    list-style-position: initial;
-  }
+details > summary:first-of-type > *|* {
+  /* Cancel "list-style-position: inside" inherited from summary. */
+  list-style-position: initial;
 }
 
 /* emulation of non-standard HTML <marquee> tag */
 marquee {
   inline-size: -moz-available;
   display: inline-block;
   vertical-align: text-bottom;
   text-align: start;

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5422,19 +5422,16 @@ pref("narrate.filter-voices", true);
 // Whether to allow, on a Linux system that doesn't support the necessary sandboxing
 // features, loading Gecko Media Plugins unsandboxed.  However, EME CDMs will not be
 // loaded without sandboxing even if this pref is changed.
 pref("media.gmp.insecure.allow", false);
 #endif
 
 pref("dom.audiochannel.mutedByDefault", false);
 
-// Enable <details> and <summary> tags.
-pref("dom.details_element.enabled", true);
-
 // Secure Element API
 #ifdef MOZ_SECUREELEMENT
 pref("dom.secureelement.enabled", false);
 #endif
 
 // Allow control characters appear in composition string.
 // When this is false, control characters except
 // CHARACTER TABULATION (horizontal tab) are removed from

--- a/testing/web-platform/meta/html/dom/interfaces.html.ini
+++ b/testing/web-platform/meta/html/dom/interfaces.html.ini
@@ -1,11 +1,11 @@
 [interfaces.html]
   type: testharness
-  prefs: [dom.forms.inputmode:true, dom.details_element.enabled:true]
+  prefs: [dom.forms.inputmode:true]
   [Document interface: attribute domain]
     expected: FAIL
 
   [Document interface: attribute cookie]
     expected: FAIL
 
   [Document interface: attribute body]
     expected: FAIL

--- a/testing/web-platform/meta/html/dom/reflection-misc.html.ini
+++ b/testing/web-platform/meta/html/dom/reflection-misc.html.ini
@@ -1,11 +1,10 @@
 [reflection-misc.html]
   type: testharness
-  prefs: [dom.details_element.enabled:true]
   [html.tabIndex: setAttribute() to object "3" followed by getAttribute()]
     expected: FAIL
 
   [html.tabIndex: setAttribute() to object "3" followed by IDL get]
     expected: FAIL
 
   [script.tabIndex: setAttribute() to object "3" followed by getAttribute()]
     expected: FAIL

deleted file mode 100644
--- a/testing/web-platform/meta/html/semantics/interactive-elements/the-details-element/details.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[details.html]
-  type: testharness
-  prefs: [dom.details_element.enabled:true]

deleted file mode 100644
--- a/testing/web-platform/meta/html/semantics/interactive-elements/the-details-element/toggleEvent.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[toggleEvent.html]
-  type: testharness
-  prefs: [dom.details_element.enabled:true]

--- a/testing/web-platform/meta/html/semantics/interfaces.html.ini
+++ b/testing/web-platform/meta/html/semantics/interfaces.html.ini
@@ -1,11 +1,10 @@
 [interfaces.html]
   type: testharness
-  prefs: [dom.details_element.enabled:true]
   [Interfaces for image]
     expected: FAIL
 
   [Interfaces for keygen]
     expected: FAIL
 
   [Interfaces for marquee]
     expected: FAIL

--- a/testing/web-platform/meta/svg/interfaces.html.ini
+++ b/testing/web-platform/meta/svg/interfaces.html.ini
@@ -1,11 +1,10 @@
 [interfaces.html]
   type: testharness
-  prefs: [dom.details_element.enabled:true]
   [SVGGeometryElement interface: existence and properties of interface object]
     expected: FAIL
 
   [SVGGeometryElement interface object length]
     expected: FAIL
 
   [SVGGeometryElement interface object name]
     expected: FAIL