--- a/dom/media/DecoderDoctorDiagnostics.cpp
+++ b/dom/media/DecoderDoctorDiagnostics.cpp
@@ -12,16 +12,17 @@
 #include "nsContentUtils.h"
 #include "nsGkAtoms.h"
 #include "nsIDocument.h"
 #include "nsIObserverService.h"
 #include "nsIScriptError.h"
 #include "nsITimer.h"
 #include "nsIWeakReference.h"
 #include "nsPluginHost.h"
+#include "nsPrintfCString.h"
 #include "VideoUtils.h"
 
 #if defined(XP_WIN)
 #include "mozilla/WindowsVersion.h"
 #endif
 
 static mozilla::LazyLogModule sDecoderDoctorLog("DecoderDoctor");
 #define DD_LOG(level, arg, ...) MOZ_LOG(sDecoderDoctorLog, level, (arg, ##__VA_ARGS__))
@@ -467,16 +468,20 @@ DecoderDoctorDocumentWatcher::Synthesize
                               diag.mDecoderDoctorDiagnostics.KeySystem());
           DecoderDoctorDiagnostics::KeySystemIssue issue =
             diag.mDecoderDoctorDiagnostics.GetKeySystemIssue();
           if (issue != DecoderDoctorDiagnostics::eUnset) {
             lastKeySystemIssue = issue;
           }
         }
         break;
+      case DecoderDoctorDiagnostics::eEvent:
+        // Events shouldn't be stored for processing.
+        MOZ_ASSERT(false);
+        break;
       default:
         MOZ_ASSERT(diag.mDecoderDoctorDiagnostics.Type()
                      == DecoderDoctorDiagnostics::eFormatSupportCheck
                    || diag.mDecoderDoctorDiagnostics.Type()
                         == DecoderDoctorDiagnostics::eMediaKeySystemAccessRequest);
         break;
     }
   }
@@ -599,25 +604,25 @@ DecoderDoctorDocumentWatcher::Synthesize
            this, mDocument);
 }
 
 void
 DecoderDoctorDocumentWatcher::AddDiagnostics(DecoderDoctorDiagnostics&& aDiagnostics,
                                              const char* aCallSite)
 {
   MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aDiagnostics.Type() != DecoderDoctorDiagnostics::eEvent);
 
   if (!mDocument) {
     return;
   }
 
   DD_DEBUG("DecoderDoctorDocumentWatcher[%p, doc=%p]::AddDiagnostics(DecoderDoctorDiagnostics{%s}, call site '%s')",
            this, mDocument, aDiagnostics.GetDescription().Data(), aCallSite);
-  mDiagnosticsSequence.AppendElement(
-    Diagnostics(Move(aDiagnostics), aCallSite));
+  mDiagnosticsSequence.AppendElement(Diagnostics(Move(aDiagnostics), aCallSite));
   EnsureTimerIsStarted();
 }
 
 NS_IMETHODIMP
 DecoderDoctorDocumentWatcher::Notify(nsITimer* timer)
 {
   MOZ_ASSERT(NS_IsMainThread());
   MOZ_ASSERT(timer == mTimer);
@@ -722,29 +727,56 @@ DecoderDoctorDiagnostics::StoreMediaKeyS
     DD_WARN("DecoderDoctorDiagnostics[%p]::StoreMediaKeySystemAccess(nsIDocument* aDocument=%p, keysystem='%s', supported=%d, call site '%s') - Could not create document watcher",
             this, aDocument, NS_ConvertUTF16toUTF8(aKeySystem).get(), aIsSupported, aCallSite);
     return;
   }
 
   mKeySystem = aKeySystem;
   mIsKeySystemSupported = aIsSupported;
 
-  // StoreDiagnostics should only be called once, after all data is available,
-  // so it is safe to Move() from this object.
+  // StoreMediaKeySystemAccess should only be called once, after all data is
+  // available, so it is safe to Move() from this object.
   watcher->AddDiagnostics(Move(*this), aCallSite);
   // Even though it's moved-from, the type should stay set
   // (Only used to ensure that we do store only once.)
   MOZ_ASSERT(mDiagnosticsType == eMediaKeySystemAccessRequest);
 }
 
+void
+DecoderDoctorDiagnostics::StoreEvent(nsIDocument* aDocument,
+                                     const DecoderDoctorEvent& aEvent,
+                                     const char* aCallSite)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  // Make sure we only store once.
+  MOZ_ASSERT(mDiagnosticsType == eUnsaved);
+  mDiagnosticsType = eEvent;
+  mEvent = aEvent;
+
+  if (NS_WARN_IF(!aDocument)) {
+    DD_WARN("DecoderDoctorDiagnostics[%p]::StoreEvent(nsIDocument* aDocument=nullptr, aEvent=%s, call site '%s')",
+            this, GetDescription().get(), aCallSite);
+    return;
+  }
+
+  // TODO: Handle event here.
+}
+
+static const char*
+EventDomainString(DecoderDoctorEvent::Domain aDomain)
+{
+  switch (aDomain) {
+    // TODO
+  }
+  return "?";
+}
+
 nsCString
 DecoderDoctorDiagnostics::GetDescription() const
 {
-  MOZ_ASSERT(mDiagnosticsType == eFormatSupportCheck
-             || mDiagnosticsType == eMediaKeySystemAccessRequest);
   nsCString s;
   switch (mDiagnosticsType) {
     case eUnsaved:
       s = "Unsaved diagnostics, cannot get accurate description";
       break;
     case eFormatSupportCheck:
       s = "format='";
       s += NS_ConvertUTF16toUTF8(mFormat).get();
@@ -776,16 +808,21 @@ DecoderDoctorDiagnostics::GetDescription
       switch (mKeySystemIssue) {
         case eUnset:
           break;
         case eWidevineWithNoWMF:
           s += ", Widevine with no WMF";
           break;
       }
       break;
+    case eEvent:
+      s = nsPrintfCString("event domain %s result=%u",
+                          EventDomainString(mEvent.mDomain), mEvent.mResult);
+      break;
     default:
+      MOZ_ASSERT_UNREACHABLE("Unexpected DiagnosticsType");
       s = "?";
       break;
   }
   return s;
 }
 
 } // namespace mozilla

--- a/dom/media/DecoderDoctorDiagnostics.h
+++ b/dom/media/DecoderDoctorDiagnostics.h
@@ -8,16 +8,23 @@
 #define DecoderDoctorDiagnostics_h_
 
 #include "nsString.h"
 
 class nsIDocument;
 
 namespace mozilla {
 
+struct DecoderDoctorEvent {
+  enum Domain {
+    // TODO
+  } mDomain;
+  nsresult mResult;
+};
+
 // DecoderDoctorDiagnostics class, used to gather data from PDMs/DecoderTraits,
 // and then notify the user about issues preventing (or worsening) playback.
 //
 // The expected usage is:
 // 1. Instantiate a DecoderDoctorDiagnostics in a function (close to the point
 //    where a webpage is trying to know whether some MIME types can be played,
 //    or trying to play a media file).
 // 2. Pass a pointer to the DecoderDoctorDiagnostics structure to one of the
@@ -41,20 +48,25 @@ public:
                               bool aCanPlay,
                               const char* aCallSite);
 
   void StoreMediaKeySystemAccess(nsIDocument* aDocument,
                                  const nsAString& aKeySystem,
                                  bool aIsSupported,
                                  const char* aCallSite);
 
+  void StoreEvent(nsIDocument* aDocument,
+                  const DecoderDoctorEvent& aEvent,
+                  const char* aCallSite);
+
   enum DiagnosticsType {
     eUnsaved,
     eFormatSupportCheck,
-    eMediaKeySystemAccessRequest
+    eMediaKeySystemAccessRequest,
+    eEvent
   };
   DiagnosticsType Type() const { return mDiagnosticsType; }
 
   // Description string, for logging purposes; only call on stored diags.
   nsCString GetDescription() const;
 
   // Methods to record diagnostic information:
 
@@ -86,16 +98,21 @@ public:
   {
     mKeySystemIssue = aKeySystemIssue;
   }
   KeySystemIssue GetKeySystemIssue() const
   {
     return mKeySystemIssue;
   }
 
+  DecoderDoctorEvent event() const
+  {
+    return mEvent;
+  }
+
 private:
   // Currently-known type of diagnostics. Set from one of the 'Store...' methods.
   // This helps ensure diagnostics are only stored once,
   // and makes it easy to know what information they contain.
   DiagnosticsType mDiagnosticsType = eUnsaved;
 
   nsString mFormat;
   // True if there is at least one decoder that can play that format.
@@ -106,13 +123,15 @@ private:
   bool mGMPPDMFailedToStartup = false;
   bool mVideoNotSupported = false;
   bool mAudioNotSupported = false;
   nsCString mGMP;
 
   nsString mKeySystem;
   bool mIsKeySystemSupported = false;
   KeySystemIssue mKeySystemIssue = eUnset;
+
+  DecoderDoctorEvent mEvent;
 };
 
 } // namespace mozilla
 
 #endif