diff --git a/accessible/tests/mochitest/elm/test_HTMLSpec.html b/accessible/tests/mochitest/elm/test_HTMLSpec.html
index f5f48ec56731..4e91784b0353 100644
--- a/accessible/tests/mochitest/elm/test_HTMLSpec.html
+++ b/accessible/tests/mochitest/elm/test_HTMLSpec.html
@@ -421,7 +421,7 @@
       //////////////////////////////////////////////////////////////////////////
       // HTML:dialog
 
-      todo(isAccessible("dialog"), "dialog element is not accessible");
+      ok(isAccessible("dialog"), "dialog element is not accessible");
 
       //////////////////////////////////////////////////////////////////////////
       // HTML:div
diff --git a/dom/html/HTMLDialogElement.cpp b/dom/html/HTMLDialogElement.cpp
new file mode 100644
index 000000000000..1f796f9a6c8f
--- /dev/null
+++ b/dom/html/HTMLDialogElement.cpp
@@ -0,0 +1,65 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/HTMLDialogElement.h"
+#include "mozilla/dom/HTMLDialogElementBinding.h"
+
+NS_IMPL_NS_NEW_HTML_ELEMENT(Dialog)
+
+namespace mozilla {
+namespace dom {
+
+HTMLDialogElement::~HTMLDialogElement()
+{
+}
+
+NS_IMPL_ELEMENT_CLONE(HTMLDialogElement)
+
+void
+HTMLDialogElement::Close(const mozilla::dom::Optional<nsAString>& aReturnValue)
+{
+  if (!Open()) {
+    return;
+  }
+  if (aReturnValue.WasPassed()) {
+    SetReturnValue(aReturnValue.Value());
+  }
+  ErrorResult ignored;
+  SetOpen(false, ignored);
+  ignored.SuppressException();
+}
+
+void
+HTMLDialogElement::Show()
+{
+  if (Open()) {
+    return;
+  }
+  ErrorResult ignored;
+  SetOpen(true, ignored);
+  ignored.SuppressException();
+}
+
+void
+HTMLDialogElement::ShowModal(ErrorResult& aError)
+{
+  if (!IsInComposedDoc() || Open()) {
+   aError.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
+   return;
+  }
+
+  SetOpen(true, aError);
+  aError.SuppressException();
+}
+
+JSObject*
+HTMLDialogElement::WrapNode(JSContext* aCx, JS::Handle<JSObject*> aGivenProto)
+{
+  return HTMLDialogElementBinding::Wrap(aCx, this, aGivenProto);
+}
+
+} // namespace dom
+} // namespace mozilla
diff --git a/dom/html/HTMLDialogElement.h b/dom/html/HTMLDialogElement.h
new file mode 100644
index 000000000000..7abaee81ca0b
--- /dev/null
+++ b/dom/html/HTMLDialogElement.h
@@ -0,0 +1,58 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef HTMLDialogElement_h
+#define HTMLDialogElement_h
+
+#include "mozilla/Attributes.h"
+#include "nsGenericHTMLElement.h"
+#include "nsGkAtoms.h"
+
+namespace mozilla {
+namespace dom {
+
+class HTMLDialogElement final : public nsGenericHTMLElement
+{
+public:
+  explicit HTMLDialogElement(already_AddRefed<mozilla::dom::NodeInfo>& aNodeInfo) : nsGenericHTMLElement(aNodeInfo)
+  {
+  }
+
+  NS_IMPL_FROMCONTENT_HTML_WITH_TAG(HTMLDialogElement, dialog)
+
+  virtual nsresult Clone(mozilla::dom::NodeInfo* aNodeInfo, nsINode** aResult) const override;
+
+  bool Open() const { return GetBoolAttr(nsGkAtoms::open); }
+  void SetOpen(bool aOpen, ErrorResult& aError)
+  {
+    SetHTMLBoolAttr(nsGkAtoms::open, aOpen, aError);
+  }
+
+  void GetReturnValue(nsAString& aReturnValue)
+  {
+    aReturnValue = mReturnValue;
+  }
+  void SetReturnValue(const nsAString& aReturnValue)
+  {
+    mReturnValue = aReturnValue;
+  }
+
+  void Close(const mozilla::dom::Optional<nsAString>& aReturnValue);
+  void Show();
+  void ShowModal(ErrorResult& aError);
+
+  nsString mReturnValue;
+
+protected:
+  virtual ~HTMLDialogElement();
+  JSObject* WrapNode(JSContext* aCx,
+                     JS::Handle<JSObject*> aGivenProto) override;
+};
+
+} // namespace dom
+} // namespace mozilla
+
+#endif
diff --git a/dom/html/moz.build b/dom/html/moz.build
index dcbe9cf3b914..d6e27c69fae3 100644
--- a/dom/html/moz.build
+++ b/dom/html/moz.build
@@ -55,6 +55,7 @@ EXPORTS.mozilla.dom += [
     'HTMLDataElement.h',
     'HTMLDataListElement.h',
     'HTMLDetailsElement.h',
+    'HTMLDialogElement.h',
     'HTMLDivElement.h',
     'HTMLFieldSetElement.h',
     'HTMLFontElement.h',
@@ -133,6 +134,7 @@ UNIFIED_SOURCES += [
     'HTMLDataElement.cpp',
     'HTMLDataListElement.cpp',
     'HTMLDetailsElement.cpp',
+    'HTMLDialogElement.cpp',
     'HTMLDivElement.cpp',
     'HTMLElement.cpp',
     'HTMLFieldSetElement.cpp',
diff --git a/dom/html/nsGenericHTMLElement.h b/dom/html/nsGenericHTMLElement.h
index 2abbd89e6061..cd3f857b82f6 100644
--- a/dom/html/nsGenericHTMLElement.h
+++ b/dom/html/nsGenericHTMLElement.h
@@ -1658,6 +1658,7 @@ NS_DECLARE_NS_NEW_HTML_ELEMENT(Mod)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(Data)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(DataList)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(Details)
+NS_DECLARE_NS_NEW_HTML_ELEMENT(Dialog)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(Div)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(FieldSet)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(Font)
diff --git a/dom/tests/mochitest/general/test_interfaces.html b/dom/tests/mochitest/general/test_interfaces.html
index 0123040ac859..45ca69b6b6c6 100644
--- a/dom/tests/mochitest/general/test_interfaces.html
+++ b/dom/tests/mochitest/general/test_interfaces.html
@@ -446,6 +446,8 @@ var interfaceNamesInGlobalScope =
     "HTMLDataListElement",
 // IMPORTANT: Do not change this list without review from a DOM peer!
     "HTMLDetailsElement",
+// IMPORTANT: Do not change this list without review from a DOM peer!
+    "HTMLDialogElement",
 // IMPORTANT: Do not change this list without review from a DOM peer!
     "HTMLDirectoryElement",
 // IMPORTANT: Do not change this list without review from a DOM peer!
diff --git a/dom/webidl/HTMLDialogElement.webidl b/dom/webidl/HTMLDialogElement.webidl
new file mode 100644
index 000000000000..f4aa8484abf8
--- /dev/null
+++ b/dom/webidl/HTMLDialogElement.webidl
@@ -0,0 +1,22 @@
+/* -*- Mode: IDL; 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/.
+ *
+ * The origin of this IDL file is
+ * https://html.spec.whatwg.org/multipage/forms.html#the-dialog-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.
+ */
+
+interface HTMLDialogElement : HTMLElement {
+  [SetterThrows] attribute boolean open;
+  attribute DOMString returnValue;
+
+  void show();
+  [Throws] void showModal();
+
+  void close(optional DOMString returnValue);
+};
diff --git a/dom/webidl/moz.build b/dom/webidl/moz.build
index 3d8a6f7c9c85..5c1d8f1646b1 100644
--- a/dom/webidl/moz.build
+++ b/dom/webidl/moz.build
@@ -189,6 +189,7 @@ WEBIDL_FILES = [
     'HTMLDataElement.webidl',
     'HTMLDataListElement.webidl',
     'HTMLDetailsElement.webidl',
+    'HTMLDialogElement.webidl',
     'HTMLDirectoryElement.webidl',
     'HTMLDivElement.webidl',
     'HTMLDListElement.webidl',
diff --git a/editor/libeditor/HTMLEditUtils.cpp b/editor/libeditor/HTMLEditUtils.cpp
index a701c06ec0ac..0adc5d51134f 100644
--- a/editor/libeditor/HTMLEditUtils.cpp
+++ b/editor/libeditor/HTMLEditUtils.cpp
@@ -517,9 +517,9 @@ HTMLEditUtils::SupportsAlignAttr(nsIDOMNode* aNode)
 #define GROUP_FORMCONTROL      (1 << 6)
 
 // address, applet, article, aside, blockquote, button, center, del, details,
-// dir, div, dl, fieldset, figure, footer, form, h1, h2, h3, h4, h5, h6, header,
-// hgroup, hr, iframe, ins, main, map, menu, nav, noframes, noscript, object,
-// ol, p, pre, table, section, summary, ul
+// dialog, dir, div, dl, fieldset, figure, footer, form, h1, h2, h3, h4, h5,
+// h6, header, hgroup, hr, iframe, ins, main, map, menu, nav, noframes,
+// noscript, object, ol, p, pre, table, section, summary, ul
 #define GROUP_BLOCK            (1 << 7)
 
 // frame, frameset
@@ -638,6 +638,7 @@ static const ElementInfo kElements[eHTMLTag_userdefined] = {
   ELEM(del, true, true, GROUP_PHRASE | GROUP_BLOCK, GROUP_FLOW_ELEMENT),
   ELEM(details, true, true, GROUP_BLOCK, GROUP_FLOW_ELEMENT),
   ELEM(dfn, true, true, GROUP_PHRASE, GROUP_INLINE_ELEMENT),
+  ELEM(dialog, true, true, GROUP_BLOCK, GROUP_FLOW_ELEMENT),
   ELEM(dir, true, false, GROUP_BLOCK, GROUP_LI),
   ELEM(div, true, true, GROUP_BLOCK, GROUP_FLOW_ELEMENT),
   ELEM(dl, true, false, GROUP_BLOCK, GROUP_DL_CONTENT),
diff --git a/layout/style/res/html.css b/layout/style/res/html.css
index 4f52ba7d96a2..e4c99ba62c3d 100644
--- a/layout/style/res/html.css
+++ b/layout/style/res/html.css
@@ -798,6 +798,27 @@ details > summary:first-of-type > *|* {
   list-style-position: initial;
 }
 
+/* <dialog> element styles */
+
+dialog {
+  position: absolute;
+  offset-inline-start: 0;
+  offset-inline-end: 0;
+  color: black;
+  margin: auto;
+  border-width: initial;
+  border-style: solid;
+  border-color: initial;
+  border-image: initial;
+  padding: 1em;
+  background: white;
+  width: -moz-fit-content;
+}
+
+dialog:not([open]) {
+  display: none;
+}
+
 /* emulation of non-standard HTML <marquee> tag */
 marquee {
   inline-size: -moz-available;
diff --git a/parser/htmlparser/nsElementTable.cpp b/parser/htmlparser/nsElementTable.cpp
index 80f581a1694c..5c69726d1f22 100644
--- a/parser/htmlparser/nsElementTable.cpp
+++ b/parser/htmlparser/nsElementTable.cpp
@@ -151,6 +151,10 @@ const nsHTMLElement gHTMLElements[] = {
     /*tag*/         eHTMLTag_dfn,
     /*parent,leaf*/ kPhrase, false
   },
+  {
+    /*tag*/         eHTMLTag_dialog,
+    /*parent,leaf*/ kBlock, false
+  },
   {
     /*tag*/         eHTMLTag_dir,
     /*parent,leaf*/ kList, false
diff --git a/parser/htmlparser/nsHTMLTagList.h b/parser/htmlparser/nsHTMLTagList.h
index 042e0c0c3376..00af46a0c61d 100644
--- a/parser/htmlparser/nsHTMLTagList.h
+++ b/parser/htmlparser/nsHTMLTagList.h
@@ -73,6 +73,7 @@ HTML_HTMLELEMENT_TAG(dd)
 HTML_TAG(del, Mod, Mod)
 HTML_TAG(details, Details, Details)
 HTML_HTMLELEMENT_TAG(dfn)
+HTML_TAG(dialog, Dialog, Dialog)
 HTML_TAG(dir, Shared, Directory)
 HTML_TAG(div, Div, Div)
 HTML_TAG(dl, SharedList, DList)