--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -8599,16 +8599,37 @@ nsDocument::EnumerateSubDocuments(nsSubD
   }
   for (auto subdoc : subdocs) {
     if (!aCallback(subdoc, aData)) {
       break;
     }
   }
 }
 
+void
+nsDocument::CollectDescendantDocuments(
+  nsTArray<nsCOMPtr<nsIDocument>>& aDescendants,
+  nsDocTestFunc aCallback) const
+{
+  if (!mSubDocuments) {
+    return;
+  }
+
+  for (auto iter = mSubDocuments->Iter(); !iter.Done(); iter.Next()) {
+    auto entry = static_cast<SubDocMapEntry*>(iter.Get());
+    const nsIDocument* subdoc = entry->mSubDocument;
+    if (subdoc) {
+      if (aCallback(subdoc)) {
+        aDescendants.AppendElement(entry->mSubDocument);
+      }
+      subdoc->CollectDescendantDocuments(aDescendants, aCallback);
+    }
+  }
+}
+
 #ifdef DEBUG_bryner
 #define DEBUG_PAGE_CACHE
 #endif
 
 bool
 nsDocument::CanSavePresentation(nsIRequest *aNewRequest)
 {
   if (EventHandlingSuppressed()) {

--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -673,16 +673,19 @@ public:
                                                nsAtom* aPrefix,
                                                int32_t aNamespaceID,
                                                const nsAString* aIs = nullptr) override;
 
   virtual void Sanitize() override;
 
   virtual void EnumerateSubDocuments(nsSubDocEnumFunc aCallback,
                                                  void *aData) override;
+  virtual void CollectDescendantDocuments(
+    nsTArray<nsCOMPtr<nsIDocument>>& aDescendants,
+    nsDocTestFunc aCallback) const override;
 
   virtual bool CanSavePresentation(nsIRequest *aNewRequest) override;
   virtual void Destroy() override;
   virtual void RemovedFromDocShell() override;
   virtual already_AddRefed<nsILayoutHistoryState> GetLayoutHistoryState() const override;
 
   virtual void BlockOnload() override;
   virtual void UnblockOnload(bool aFireSync) override;

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -1912,16 +1912,25 @@ public:
    * The enumerator callback should return true to continue enumerating, or
    * false to stop.  This will never get passed a null aDocument.
    */
   typedef bool (*nsSubDocEnumFunc)(nsIDocument *aDocument, void *aData);
   virtual void EnumerateSubDocuments(nsSubDocEnumFunc aCallback,
                                      void *aData) = 0;
 
   /**
+   * Collect all the descendant documents for which |aCalback| returns true.
+   * The callback function must not mutate any state for the given document.
+   */
+  typedef bool (*nsDocTestFunc)(const nsIDocument* aDocument);
+  virtual void CollectDescendantDocuments(
+    nsTArray<nsCOMPtr<nsIDocument>>& aDescendants,
+    nsDocTestFunc aCallback) const = 0;
+
+  /**
    * Check whether it is safe to cache the presentation of this document
    * and all of its subdocuments. This method checks the following conditions
    * recursively:
    *  - Some document types, such as plugin documents, cannot be safely cached.
    *  - If there are any pending requests, we don't allow the presentation
    *    to be cached.  Ideally these requests would be suspended and resumed,
    *    but that is difficult in some cases, such as XMLHttpRequest.
    *  - If there are any beforeunload or unload listeners, we must fire them