diff --git a/dom/html/HTMLElement.cpp b/dom/html/HTMLElement.cpp
index bccb117034ec..37daa19d4fb9 100644
--- a/dom/html/HTMLElement.cpp
+++ b/dom/html/HTMLElement.cpp
@@ -381,10 +381,6 @@ HTMLFieldSetElement* HTMLElement::GetFieldSetInternal() const {
 
 bool HTMLElement::CanBeDisabled() const { return IsFormAssociatedElement(); }
 
-bool HTMLElement::DoesReadOnlyApply() const {
-  return IsFormAssociatedElement();
-}
-
 void HTMLElement::UpdateDisabledState(bool aNotify) {
   bool oldState = IsDisabled();
   nsGenericHTMLFormElement::UpdateDisabledState(aNotify);
diff --git a/dom/html/HTMLElement.h b/dom/html/HTMLElement.h
index 727e6ab487e0..177904527f86 100644
--- a/dom/html/HTMLElement.h
+++ b/dom/html/HTMLElement.h
@@ -75,7 +75,6 @@ class HTMLElement final : public nsGenericHTMLFormElement {
   void SetFieldSetInternal(HTMLFieldSetElement* aFieldset) override;
   HTMLFieldSetElement* GetFieldSetInternal() const override;
   bool CanBeDisabled() const override;
-  bool DoesReadOnlyApply() const override;
   void UpdateDisabledState(bool aNotify) override;
   void UpdateFormOwner(bool aBindToTree, Element* aFormIdElement) override;
 
diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index 9a7f7e02bdb1..67c692084c95 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -6553,7 +6553,7 @@ HTMLInputElement::ValueModeType HTMLInputElement::GetValueMode() const {
 
 bool HTMLInputElement::IsMutable() const {
   return !IsDisabled() &&
-         !(DoesReadOnlyApply() && State().HasState(ElementState::READONLY));
+         !(DoesReadWriteApply() && State().HasState(ElementState::READONLY));
 }
 
 bool HTMLInputElement::DoesRequiredApply() const {
diff --git a/dom/html/nsGenericHTMLElement.cpp b/dom/html/nsGenericHTMLElement.cpp
index 440a7cfcb794..59d0b07aed9d 100644
--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -2233,7 +2233,7 @@ void nsGenericHTMLFormElement::UpdateDisabledState(bool aNotify) {
 
   if (!changedStates.IsEmpty()) {
     ToggleStates(changedStates, aNotify);
-    if (DoesReadOnlyApply()) {
+    if (DoesReadWriteApply()) {
       // :disabled influences :read-only / :read-write.
       UpdateReadOnlyState(aNotify);
     }
@@ -2241,7 +2241,7 @@ void nsGenericHTMLFormElement::UpdateDisabledState(bool aNotify) {
 }
 
 bool nsGenericHTMLFormElement::IsReadOnlyInternal() const {
-  if (DoesReadOnlyApply()) {
+  if (DoesReadWriteApply()) {
     return IsDisabled() || GetBoolAttr(nsGkAtoms::readonly);
   }
   return nsGenericHTMLElement::IsReadOnlyInternal();
@@ -2682,7 +2682,7 @@ bool nsGenericHTMLFormControlElement::CanBeDisabled() const {
   return type != FormControlType::Object && type != FormControlType::Output;
 }
 
-bool nsGenericHTMLFormControlElement::DoesReadOnlyApply() const {
+bool nsGenericHTMLFormControlElement::DoesReadWriteApply() const {
   auto type = ControlType();
   if (!IsInputElement(type) && type != FormControlType::Textarea) {
     return false;
@@ -2716,7 +2716,7 @@ bool nsGenericHTMLFormControlElement::DoesReadOnlyApply() const {
     case FormControlType::InputDatetimeLocal:
       return true;
     default:
-      MOZ_ASSERT_UNREACHABLE("Unexpected input type in DoesReadOnlyApply()");
+      MOZ_ASSERT_UNREACHABLE("Unexpected input type in DoesReadWriteApply()");
       return true;
 #else   // DEBUG
     default:
diff --git a/dom/html/nsGenericHTMLElement.h b/dom/html/nsGenericHTMLElement.h
index 31fc5e1e58cd..ea85e34dfbdf 100644
--- a/dom/html/nsGenericHTMLElement.h
+++ b/dom/html/nsGenericHTMLElement.h
@@ -1140,9 +1140,10 @@ class nsGenericHTMLFormElement : public nsGenericHTMLElement {
   virtual bool CanBeDisabled() const { return false; }
 
   /**
-   * Returns if the readonly attribute applies.
+   * Returns true if :read-write pseudo class may match the element even if the
+   * element isn't part of designMode or contenteditable.
    */
-  virtual bool DoesReadOnlyApply() const { return false; }
+  virtual bool DoesReadWriteApply() const { return false; }
 
   /**
    *  Returns true if the element is a form associated element.
@@ -1200,7 +1201,7 @@ class nsGenericHTMLFormControlElement : public nsGenericHTMLFormElement,
 
   // nsGenericHTMLFormElement
   bool CanBeDisabled() const override;
-  bool DoesReadOnlyApply() const override;
+  bool DoesReadWriteApply() const override;
   void SetFormInternal(mozilla::dom::HTMLFormElement* aForm,
                        bool aBindToTree) override;
   mozilla::dom::HTMLFormElement* GetFormInternal() const override;
diff --git a/testing/web-platform/tests/html/semantics/selectors/pseudo-classes/readwrite-readonly.html b/testing/web-platform/tests/html/semantics/selectors/pseudo-classes/readwrite-readonly.html
index fb8a5b9ad70e..ad579ccc0eaf 100644
--- a/testing/web-platform/tests/html/semantics/selectors/pseudo-classes/readwrite-readonly.html
+++ b/testing/web-platform/tests/html/semantics/selectors/pseudo-classes/readwrite-readonly.html
@@ -5,6 +5,17 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script src="utils.js"></script>
+<script>
+  class CustomElement extends HTMLElement {
+    static formAssociated = true;
+
+    constructor() {
+      super();
+    }
+  }
+
+  window.customElements.define("custom-element", CustomElement);
+</script>
 <div id="log"></div>
 
 <div id=set0>
@@ -58,6 +69,14 @@
   </div>
 </div>
 
+<div id=set6>
+  <custom-element id=ce1>content</custom-element>
+  <custom-element id=ce2 contenteditable>content</custom-element>
+  <custom-element id=ce3 contenteditable readonly>content</custom-element>
+  <custom-element id=ce4 contenteditable disabled>content</custom-element>
+  <custom-element id=ce5 contenteditable readonly disabled>content</custom-element>
+</div>
+
 <script>
   testSelectorIdsMatch("#set0 :read-write", [], "The :read-write pseudo-class must not match input elements to which the readonly attribute does not apply");
 
@@ -115,4 +134,9 @@
 
   testSelectorIdsMatch("#set5 :read-write", ["cd1", "p3", "ci3", "ci4", "ct3", "ct4"], "The :read-write pseudo-class must match elements that are inside editing hosts, but not match inputs and textareas inside that aren't");
 
+  testSelectorIdsMatch("#set6 :read-only", ["ce1"], "The :read-only pseudo-class must match form-associated custom elements");
+
+  testSelectorIdsMatch("#set6 :read-write", ["ce2", "ce3", "ce4", "ce5"], "The :read-write pseudo-class must match form-associated contenteditable custom elements");
+
+
 </script>