--- a/accessible/generic/DocAccessible.cpp
+++ b/accessible/generic/DocAccessible.cpp
@@ -11,16 +11,17 @@
 #include "HTMLImageMapAccessible.h"
 #include "nsAccCache.h"
 #include "nsAccessiblePivot.h"
 #include "nsAccUtils.h"
 #include "nsDeckFrame.h"
 #include "nsEventShell.h"
 #include "nsLayoutUtils.h"
 #include "nsTextEquivUtils.h"
+#include "Pivot.h"
 #include "Role.h"
 #include "RootAccessible.h"
 #include "TreeWalker.h"
 #include "xpcAccessibleDocument.h"
 
 #include "nsCommandManager.h"
 #include "nsContentUtils.h"
 #include "nsIDocShell.h"
@@ -846,25 +847,60 @@ void DocAccessible::AttributeChangedImpl
   // Fire name change and description change events. XXX: it's not complete and
   // dupes the code logic of accessible name and description calculation, we do
   // that for performance reasons.
   if (aAttribute == nsGkAtoms::aria_label) {
     FireDelayedEvent(nsIAccessibleEvent::EVENT_NAME_CHANGE, aAccessible);
     return;
   }
 
+  dom::Element* elm = aAccessible->GetContent()->AsElement();
   if (aAttribute == nsGkAtoms::aria_describedby) {
     FireDelayedEvent(nsIAccessibleEvent::EVENT_DESCRIPTION_CHANGE, aAccessible);
+    if (aModType == dom::MutationEvent_Binding::MODIFICATION ||
+        aModType == dom::MutationEvent_Binding::ADDITION) {
+      // The subtrees of the new aria-describedby targets might be used to
+      // compute the description for aAccessible. Therefore, we need to set
+      // the eHasDescriptionDependent flag on all Accessibles in these subtrees.
+      IDRefsIterator iter(this, aAccessible->Elm(),
+                          nsGkAtoms::aria_describedby);
+      while (Accessible* target = iter.Next()) {
+        Pivot pivot(target);
+        LocalAccInSameDocRule rule;
+        for (AccessibleOrProxy anchor(target); !anchor.IsNull();
+             anchor = pivot.Next(anchor, rule)) {
+          Accessible* acc = anchor.AsAccessible();
+          MOZ_ASSERT(acc);
+          acc->mContextFlags |= eHasDescriptionDependent;
+        }
+      }
+    }
     return;
   }
 
-  dom::Element* elm = aAccessible->GetContent()->AsElement();
   if (aAttribute == nsGkAtoms::aria_labelledby &&
       !elm->HasAttr(kNameSpaceID_None, nsGkAtoms::aria_label)) {
     FireDelayedEvent(nsIAccessibleEvent::EVENT_NAME_CHANGE, aAccessible);
+    if (aModType == dom::MutationEvent_Binding::MODIFICATION ||
+        aModType == dom::MutationEvent_Binding::ADDITION) {
+      // The subtrees of the new aria-labelledby targets might be used to
+      // compute the name for aAccessible. Therefore, we need to set
+      // the eHasNameDependent flag on all Accessibles in these subtrees.
+      IDRefsIterator iter(this, aAccessible->Elm(), nsGkAtoms::aria_labelledby);
+      while (Accessible* target = iter.Next()) {
+        Pivot pivot(target);
+        LocalAccInSameDocRule rule;
+        for (AccessibleOrProxy anchor(target); !anchor.IsNull();
+             anchor = pivot.Next(anchor, rule)) {
+          Accessible* acc = anchor.AsAccessible();
+          MOZ_ASSERT(acc);
+          acc->mContextFlags |= eHasNameDependent;
+        }
+      }
+    }
     return;
   }
 
   if (aAttribute == nsGkAtoms::alt &&
       !elm->HasAttr(kNameSpaceID_None, nsGkAtoms::aria_label) &&
       !elm->HasAttr(kNameSpaceID_None, nsGkAtoms::aria_labelledby)) {
     FireDelayedEvent(nsIAccessibleEvent::EVENT_NAME_CHANGE, aAccessible);
     return;

--- a/accessible/tests/mochitest/events/test_descrchange.html
+++ b/accessible/tests/mochitest/events/test_descrchange.html
@@ -85,16 +85,32 @@
       descChanged = PromEvents.waitForEvent(
         EVENT_DESCRIPTION_CHANGE,
         describedBy
       );
       info("Changing text of aria-describedby target's child");
       getNode("descriptionChild").textContent = "d4";
       await descChanged;
 
+      const lateDescribedBy = getNode("lateDescribedBy");
+      descChanged = PromEvents.waitForEvent(
+        EVENT_DESCRIPTION_CHANGE,
+        lateDescribedBy
+      );
+      info("Setting aria-describedby");
+      lateDescribedBy.setAttribute("aria-describedby", "lateDescription");
+      await descChanged;
+      descChanged = PromEvents.waitForEvent(
+        EVENT_DESCRIPTION_CHANGE,
+        lateDescribedBy
+      );
+      info("Changing text of late aria-describedby target's child");
+      getNode("lateDescriptionChild").textContent = "d2";
+      await descChanged;
+
       SimpleTest.finish();
     }
 
     SimpleTest.waitForExplicitFinish();
     addA11yLoadEvent(doTests);
   </script>
 </head>
 
@@ -112,11 +128,14 @@
   </pre>
 
   <button id="tst1">btn1</button>
   <button id="tst2">btn2</button>
 
   <div id="describedBy" aria-describedby="description"></div>
   <div id="description">d1</div>
 
+  <div id="lateDescribedBy"></div>
+  <div id="lateDescription"><p id="lateDescriptionChild">d1</p></div>
+
   <div id="eventdump"></div>
 </body>
 </html>

--- a/accessible/tests/mochitest/events/test_namechange.html
+++ b/accessible/tests/mochitest/events/test_namechange.html
@@ -114,16 +114,26 @@
       info("Adding node to aria-labelledby target");
       label.innerHTML = '<p id="labelChild">l3</p>';
       await nameChanged;
       nameChanged = PromEvents.waitForEvent(EVENT_NAME_CHANGE, labelledBy);
       info("Changing text of aria-labelledby target's child");
       getNode("labelChild").textContent = "l4";
       await nameChanged;
 
+      const lateLabelledBy = getNode("lateLabelledBy");
+      nameChanged = PromEvents.waitForEvent(EVENT_NAME_CHANGE, lateLabelledBy);
+      info("Setting aria-labelledby");
+      lateLabelledBy.setAttribute("aria-labelledby", "lateLabel");
+      await nameChanged;
+      nameChanged = PromEvents.waitForEvent(EVENT_NAME_CHANGE, lateLabelledBy);
+      info("Changing text of late aria-labelledby target's child");
+      getNode("lateLabelChild").textContent = "l2";
+      await nameChanged;
+
       SimpleTest.finish();
     }
 
     SimpleTest.waitForExplicitFinish();
     addA11yLoadEvent(doTests);
   </script>
 </head>
 
@@ -143,11 +153,14 @@
   <img id="tst1" alt="initial" src="../moz.png">
   <img id="tst2" src="../moz.png">
   <img id="tst3">
   <img id="tst4" src="../moz.png">
 
   <div id="labelledBy" aria-labelledby="label"></div>
   <div id="label">l1</div>
 
+  <div id="lateLabelledBy"></div>
+  <div id="lateLabel"><p id="lateLabelChild">l1</p></div>
+
   <div id="eventdump"></div>
 </body>
 </html>