diff --git a/dom/html/nsGenericHTMLElement.cpp b/dom/html/nsGenericHTMLElement.cpp
index bd17b8554587..c406c2ba3420 100644
--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -269,6 +269,76 @@ void nsGenericHTMLElement::GetAccessKeyLabel(nsString& aLabel) {
   }
 }
 
+// https://html.spec.whatwg.org/#dom-hidden
+void nsGenericHTMLElement::GetHidden(
+    Nullable<OwningBooleanOrUnrestrictedDoubleOrString>& aHidden) const {
+      OwningBooleanOrUnrestrictedDoubleOrString value;
+      // 1. If the hidden attribute is in the hidden until found state, then
+      //    return "until-found".
+  nsAutoString result;
+  if (GetAttr(kNameSpaceID_None, nsGkAtoms::hidden, result)) {
+    if (StaticPrefs::dom_hidden_until_found_enabled() &&
+        result.LowerCaseEqualsLiteral("until-found")) {
+      value.SetStringLiteral(u"until-found");
+    } else {
+      // 2. If the hidden attribute is set, then return true.
+      value.SetAsBoolean() = true;
+    }
+  } else {
+    // 3. Return false.
+    value.SetAsBoolean() = false;
+  }
+
+  aHidden.SetValue(value);
+}
+
+// https://html.spec.whatwg.org/#dom-hidden
+void nsGenericHTMLElement::SetHidden(
+    const Nullable<BooleanOrUnrestrictedDoubleOrString>& aHidden,
+    ErrorResult& aRv) {
+  // 4. Otherwise, if the given value is null, then remove the hidden attribute.
+  if (aHidden.IsNull()) {
+    return UnsetAttr(nsGkAtoms::hidden, aRv);
+  }
+  bool isHidden = true;
+  const auto& value = aHidden.Value();
+  // 1. If the given value is a string that is an ASCII case-insensitive match
+  //    for "until-found", then set the hidden attribute to "until-found".
+  if (value.IsString()) {
+    const nsAString& stringValue = value.GetAsString();
+    // 3. Otherwise, if the given value is the empty string, then remove the
+    //    hidden attribute.
+    if (stringValue.IsEmpty()) {
+      isHidden = false;
+    } else if (StaticPrefs::dom_hidden_until_found_enabled() &&
+               stringValue.LowerCaseEqualsLiteral("until-found")) {
+      return SetAttr(nsGkAtoms::hidden, u"until-found"_ns, aRv);
+    }
+  }
+  // 2. Otherwise, if the given value is false, then remove the hidden
+  //    attribute.
+  else if (value.IsBoolean()) {
+    if (!value.GetAsBoolean()) {
+      isHidden = false;
+    }
+  }
+  // 5. Otherwise, if the given value is 0, then remove the hidden attribute.
+  // 6. Otherwise, if the given value is NaN, then remove the hidden attribute.
+  else if (value.IsUnrestrictedDouble()) {
+    double d = value.GetAsUnrestrictedDouble();
+    if (d == 0.0 || std::isnan(d)) {
+      isHidden = false;
+    }
+  }
+
+  // 7. Otherwise, set the hidden attribute to the empty string.
+  if (isHidden) {
+    aRv = SetAttr(kNameSpaceID_None, nsGkAtoms::hidden, u""_ns, true);
+  } else {
+    aRv = UnsetAttr(kNameSpaceID_None, nsGkAtoms::hidden, true);
+  }
+}
+
 static bool IsOffsetParent(nsIFrame* aFrame) {
   LayoutFrameType frameType = aFrame->Type();
 
diff --git a/dom/html/nsGenericHTMLElement.h b/dom/html/nsGenericHTMLElement.h
index 66ec09e6daf0..43276ff6840c 100644
--- a/dom/html/nsGenericHTMLElement.h
+++ b/dom/html/nsGenericHTMLElement.h
@@ -14,6 +14,7 @@
 #include "nsContentCreatorFunctions.h"
 #include "nsStyledElement.h"
 #include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/HTMLElementBinding.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/DOMRect.h"
 #include "mozilla/dom/ValidityState.h"
@@ -95,10 +96,16 @@ class nsGenericHTMLElement : public nsGenericHTMLElementBase {
   void SetPopover(const nsAString& aPopover, mozilla::ErrorResult& aError) {
     SetOrRemoveNullableStringAttr(nsGkAtoms::popover, aPopover, aError);
   }
-  bool Hidden() const { return GetBoolAttr(nsGkAtoms::hidden); }
-  void SetHidden(bool aHidden, mozilla::ErrorResult& aError) {
-    SetHTMLBoolAttr(nsGkAtoms::hidden, aHidden, aError);
-  }
+
+  void GetHidden(mozilla::dom::Nullable<
+                 mozilla::dom::OwningBooleanOrUnrestrictedDoubleOrString>&
+                     aHidden) const;
+
+  void SetHidden(
+      const mozilla::dom::Nullable<
+          mozilla::dom::BooleanOrUnrestrictedDoubleOrString>& aHidden,
+      mozilla::ErrorResult& aRv);
+
   bool Inert() const { return GetBoolAttr(nsGkAtoms::inert); }
   void SetInert(bool aInert, mozilla::ErrorResult& aError) {
     SetHTMLBoolAttr(nsGkAtoms::inert, aInert, aError);
diff --git a/dom/webidl/HTMLElement.webidl b/dom/webidl/HTMLElement.webidl
index c0922f7b7a98..a112fec8e899 100644
--- a/dom/webidl/HTMLElement.webidl
+++ b/dom/webidl/HTMLElement.webidl
@@ -33,7 +33,7 @@ interface HTMLElement : Element {
 
   // user interaction
   [CEReactions, SetterThrows, Pure]
-           attribute boolean hidden;
+           attribute (boolean or unrestricted double or DOMString)? hidden;
   [CEReactions, SetterThrows, Pure]
            attribute boolean inert;
   [NeedsCallerType]
diff --git a/testing/web-platform/meta/html/editing/the-hidden-attribute/__dir__.ini b/testing/web-platform/meta/html/editing/the-hidden-attribute/__dir__.ini
new file mode 100644
index 000000000000..a15e737e44c3
--- /dev/null
+++ b/testing/web-platform/meta/html/editing/the-hidden-attribute/__dir__.ini
@@ -0,0 +1 @@
+prefs: [dom.hidden_until_found.enabled:true]
\ No newline at end of file
diff --git a/testing/web-platform/meta/html/editing/the-hidden-attribute/hidden-idl.html.ini b/testing/web-platform/meta/html/editing/the-hidden-attribute/hidden-idl.html.ini
deleted file mode 100644
index 3e5a92d47b2f..000000000000
--- a/testing/web-platform/meta/html/editing/the-hidden-attribute/hidden-idl.html.ini
+++ /dev/null
@@ -1,9 +0,0 @@
-[hidden-idl.html]
-  [div.hidden = "until-found"]
-    expected: FAIL
-
-  [div.hidden = "UNTIL-FOUND"]
-    expected: FAIL
-
-  [div.hidden = "UnTiL-FoUnD"]
-    expected: FAIL
diff --git a/testing/web-platform/meta/html/editing/the-hidden-attribute/hidden-until-found-007.html.ini b/testing/web-platform/meta/html/editing/the-hidden-attribute/hidden-until-found-007.html.ini
deleted file mode 100644
index ea525afbaf24..000000000000
--- a/testing/web-platform/meta/html/editing/the-hidden-attribute/hidden-until-found-007.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[hidden-until-found-007.html]
-  expected: FAIL
diff --git a/testing/web-platform/meta/html/editing/the-hidden-attribute/hidden-until-found-idl.html.ini b/testing/web-platform/meta/html/editing/the-hidden-attribute/hidden-until-found-idl.html.ini
deleted file mode 100644
index 1505f47bcd43..000000000000
--- a/testing/web-platform/meta/html/editing/the-hidden-attribute/hidden-until-found-idl.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[hidden-until-found-idl.html]
-  [element.hidden should return "until-found" regardless of uppercase letters.]
-    expected: FAIL
diff --git a/xpcom/ds/StaticAtoms.py b/xpcom/ds/StaticAtoms.py
index e060f6cd2009..26af767ade95 100644
--- a/xpcom/ds/StaticAtoms.py
+++ b/xpcom/ds/StaticAtoms.py
@@ -1302,6 +1302,7 @@ STATIC_ATOMS = [
     Atom("u", "u"),
     Atom("ul", "ul"),
     Atom("unparsedEntityUri", "unparsed-entity-uri"),
+    Atom("untilFound", "until-found"),
     Atom("up", "up"),
     Atom("update", "update"),
     Atom("upperFirst", "upper-first"),