diff --git a/dom/base/Document.cpp b/dom/base/Document.cpp
index df81ade49f0f..92761b0b0aea 100644
--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -6692,8 +6692,7 @@ nsresult Document::LoadAdditionalStyleSheet(additionalSheetType aType,
 
   RefPtr<StyleSheet> sheet = result.unwrap();
 
-  sheet->SetAssociatedDocumentOrShadowRoot(
-      this, StyleSheet::OwnedByDocumentOrShadowRoot);
+  sheet->SetAssociatedDocumentOrShadowRoot(this);
   MOZ_ASSERT(sheet->IsApplicable());
 
   return AddAdditionalStyleSheet(aType, sheet);
diff --git a/dom/base/DocumentOrShadowRoot.cpp b/dom/base/DocumentOrShadowRoot.cpp
index 910e01183bd5..0719b23842bb 100644
--- a/dom/base/DocumentOrShadowRoot.cpp
+++ b/dom/base/DocumentOrShadowRoot.cpp
@@ -73,8 +73,7 @@ StyleSheetList* DocumentOrShadowRoot::StyleSheets() {
 }
 
 void DocumentOrShadowRoot::InsertSheetAt(size_t aIndex, StyleSheet& aSheet) {
-  aSheet.SetAssociatedDocumentOrShadowRoot(
-      this, StyleSheet::OwnedByDocumentOrShadowRoot);
+  aSheet.SetAssociatedDocumentOrShadowRoot(this);
   mStyleSheets.InsertElementAt(aIndex, &aSheet);
 }
 
diff --git a/layout/style/StyleSheet.cpp b/layout/style/StyleSheet.cpp
index c23dc1a5efac..5d0c3063d3df 100644
--- a/layout/style/StyleSheet.cpp
+++ b/layout/style/StyleSheet.cpp
@@ -40,7 +40,6 @@ StyleSheet::StyleSheet(css::SheetParsingMode aParsingMode, CORSMode aCORSMode,
       mOwnerRule(nullptr),
       mParsingMode(aParsingMode),
       mState(static_cast<State>(0)),
-      mAssociationMode(NotOwnedByDocumentOrShadowRoot),
       mInner(new StyleSheetInfo(aCORSMode, aIntegrity, aParsingMode)) {
   mInner->AddSheet(this);
 }
@@ -58,9 +57,6 @@ StyleSheet::StyleSheet(const StyleSheet& aCopy, StyleSheet* aParentSheetToUse,
       mOwnerRule(aOwnerRuleToUse),
       mParsingMode(aCopy.mParsingMode),
       mState(aCopy.mState),
-      // We only use this constructor during cloning.  It's the cloner's
-      // responsibility to notify us if we end up being owned by a document.
-      mAssociationMode(NotOwnedByDocumentOrShadowRoot),
       // Shallow copy, but concrete subclasses will fix up.
       mInner(aCopy.mInner) {
   MOZ_ASSERT(!aConstructorDocToUse || aCopy.IsConstructed());
@@ -176,8 +172,7 @@ dom::DocumentOrShadowRoot* StyleSheet::GetAssociatedDocumentOrShadowRoot()
 
 Document* StyleSheet::GetKeptAliveByDocument() const {
   for (const auto* sheet = this; sheet; sheet = sheet->mParentSheet) {
-    if (sheet->mAssociationMode == OwnedByDocumentOrShadowRoot) {
-      MOZ_ASSERT(sheet->mDocumentOrShadowRoot);
+    if (sheet->mDocumentOrShadowRoot) {
       return sheet->mDocumentOrShadowRoot->AsNode().GetComposedDoc();
     }
     if (sheet->IsConstructed()) {
@@ -884,7 +879,7 @@ void StyleSheet::RemoveFromParent() {
 }
 
 void StyleSheet::UnparentChildren() {
-  MOZ_ASSERT(mAssociationMode == NotOwnedByDocumentOrShadowRoot,
+  MOZ_ASSERT(!mDocumentOrShadowRoot,
              "How did we get to the destructor, exactly, if we're owned "
              "by a document?");
   // XXXbz this is a little bogus; see the comment where we
@@ -951,16 +946,12 @@ bool StyleSheet::AreRulesAvailable(nsIPrincipal& aSubjectPrincipal,
 }
 
 void StyleSheet::SetAssociatedDocumentOrShadowRoot(
-    DocumentOrShadowRoot* aDocOrShadowRoot, AssociationMode aAssociationMode) {
+    DocumentOrShadowRoot* aDocOrShadowRoot) {
   MOZ_ASSERT(!IsConstructed());
-  MOZ_ASSERT(aDocOrShadowRoot ||
-             aAssociationMode == NotOwnedByDocumentOrShadowRoot);
   MOZ_ASSERT(!mParentSheet || !aDocOrShadowRoot,
              "Shouldn't be set on child sheets");
-
   // not ref counted
   mDocumentOrShadowRoot = aDocOrShadowRoot;
-  mAssociationMode = aAssociationMode;
 }
 
 void StyleSheet::AppendStyleSheet(StyleSheet& aSheet) {
diff --git a/layout/style/StyleSheet.h b/layout/style/StyleSheet.h
index b63fa73afa25..e5ab7a1491d9 100644
--- a/layout/style/StyleSheet.h
+++ b/layout/style/StyleSheet.h
@@ -244,15 +244,10 @@ class StyleSheet final : public nsICSSLoaderObserver, public nsWrapperCache {
 
   void EnsureUniqueInner();
 
-  // style sheet owner info
-  enum AssociationMode : uint8_t {
-    // OwnedByDocumentOrShadowRoot means mDocumentOrShadowRoot owns us (possibly
-    // via a chain of other stylesheets).
-    OwnedByDocumentOrShadowRoot,
-    // NotOwnedByDocument means we're owned by something that might have a
-    // different lifetime than mDocument.
-    NotOwnedByDocumentOrShadowRoot
-  };
+  // Returns the DocumentOrShadowRoot* that owns us, if any.
+  //
+  // TODO(emilio): Maybe rename to GetOwner*() or such? Might be
+  // confusing with nsINode::OwnerDoc and such.
   dom::DocumentOrShadowRoot* GetAssociatedDocumentOrShadowRoot() const;
 
   // Whether this stylesheet is kept alive by the associated or constructor
@@ -267,10 +262,9 @@ class StyleSheet final : public nsICSSLoaderObserver, public nsWrapperCache {
   // Non-null iff GetAssociatedDocumentOrShadowRoot is non-null.
   dom::Document* GetAssociatedDocument() const;
 
-  void SetAssociatedDocumentOrShadowRoot(dom::DocumentOrShadowRoot*,
-                                         AssociationMode);
+  void SetAssociatedDocumentOrShadowRoot(dom::DocumentOrShadowRoot*);
   void ClearAssociatedDocumentOrShadowRoot() {
-    SetAssociatedDocumentOrShadowRoot(nullptr, NotOwnedByDocumentOrShadowRoot);
+    SetAssociatedDocumentOrShadowRoot(nullptr);
   }
 
   nsINode* GetOwnerNode() const { return mOwningNode; }
@@ -577,12 +571,6 @@ class StyleSheet final : public nsICSSLoaderObserver, public nsWrapperCache {
 
   State mState;
 
-  // mAssociationMode determines whether mDocumentOrShadowRoot directly owns us
-  // (in the sense that if it's known-live then we're known-live).
-  //
-  // Always NotOwnedByDocumentOrShadowRoot when mDocumentOrShadowRoot is null.
-  AssociationMode mAssociationMode;
-
   // Core information we get from parsed sheets, which are shared amongst
   // StyleSheet clones.
   //