--- a/dom/base/nsGkAtomList.h
+++ b/dom/base/nsGkAtomList.h
@@ -933,16 +933,18 @@ GK_ATOM(ontypechange, "ontypechange")
 GK_ATOM(onterminate, "onterminate")
 GK_ATOM(ontext, "ontext")
 GK_ATOM(ontoggle, "ontoggle")
 GK_ATOM(ontouchstart, "ontouchstart")
 GK_ATOM(ontouchend, "ontouchend")
 GK_ATOM(ontouchmove, "ontouchmove")
 GK_ATOM(ontouchcancel, "ontouchcancel")
 GK_ATOM(ontransitionend, "ontransitionend")
+GK_ATOM(ontransitionrun, "ontransitionrun")
+GK_ATOM(ontransitionstart, "ontransitionstart")
 GK_ATOM(onunderflow, "onunderflow")
 GK_ATOM(onunload, "onunload")
 GK_ATOM(onupdatefound, "onupdatefound")
 GK_ATOM(onupdateready, "onupdateready")
 GK_ATOM(onupgradeneeded, "onupgradeneeded")
 GK_ATOM(onussdreceived, "onussdreceived")
 GK_ATOM(onversionchange, "onversionchange")
 GK_ATOM(onvoicechange, "onvoicechange")

--- a/dom/events/EventNameList.h
+++ b/dom/events/EventNameList.h
@@ -974,16 +974,24 @@ NON_IDL_EVENT(MozEdgeUICanceled,
               EventNameType_None,
               eSimpleGestureEventClass)
 NON_IDL_EVENT(MozEdgeUICompleted,
               eEdgeUICompleted,
               EventNameType_None,
               eSimpleGestureEventClass)
 
 // CSS Transition & Animation events:
+EVENT(transitionstart,
+      eTransitionStart,
+      EventNameType_All,
+      eTransitionEventClass)
+EVENT(transitionrun,
+      eTransitionRun,
+      EventNameType_All,
+      eTransitionEventClass)
 EVENT(transitionend,
       eTransitionEnd,
       EventNameType_All,
       eTransitionEventClass)
 EVENT(animationstart,
       eAnimationStart,
       EventNameType_All,
       eAnimationEventClass)

--- a/dom/webidl/EventHandler.webidl
+++ b/dom/webidl/EventHandler.webidl
@@ -127,16 +127,18 @@ interface GlobalEventHandlers {
            [Pref="pointer-lock-api.prefixed.enabled"]
            attribute EventHandler onmozpointerlockerror;
 
            // CSS-Animation and CSS-Transition handlers.
            attribute EventHandler onanimationend;
            attribute EventHandler onanimationiteration;
            attribute EventHandler onanimationstart;
            attribute EventHandler ontransitionend;
+           attribute EventHandler ontransitionrun;
+           attribute EventHandler ontransitionstart;
 
            // CSS-Animation and CSS-Transition legacy handlers.
            // This handler isn't standard.
            attribute EventHandler onwebkitanimationend;
            attribute EventHandler onwebkitanimationiteration;
            attribute EventHandler onwebkitanimationstart;
            attribute EventHandler onwebkittransitionend;
 };

--- a/widget/EventMessageList.h
+++ b/widget/EventMessageList.h
@@ -329,16 +329,18 @@ NS_EVENT_MESSAGE(eContentCommandLookUpDi
 NS_EVENT_MESSAGE(eContentCommandScroll)
 
 // Event to gesture notification
 NS_EVENT_MESSAGE(eGestureNotify)
 
 NS_EVENT_MESSAGE(eScrolledAreaChanged)
 
 // CSS Transition & Animation events:
+NS_EVENT_MESSAGE(eTransitionStart)
+NS_EVENT_MESSAGE(eTransitionRun)
 NS_EVENT_MESSAGE(eTransitionEnd)
 NS_EVENT_MESSAGE(eAnimationStart)
 NS_EVENT_MESSAGE(eAnimationEnd)
 NS_EVENT_MESSAGE(eAnimationIteration)
 
 // Webkit-prefixed versions of Transition & Animation events, for web compat:
 NS_EVENT_MESSAGE(eWebkitTransitionEnd)
 NS_EVENT_MESSAGE(eWebkitAnimationStart)