--- a/dom/base/nsGkAtomList.h
+++ b/dom/base/nsGkAtomList.h
@@ -936,16 +936,17 @@ GK_ATOM(onsuccess, "onsuccess")
 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(ontransitioncancel, "ontransitioncancel")
 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")

--- a/dom/events/EventNameList.h
+++ b/dom/events/EventNameList.h
@@ -993,16 +993,20 @@ EVENT(transitionstart,
 EVENT(transitionrun,
       eTransitionRun,
       EventNameType_All,
       eTransitionEventClass)
 EVENT(transitionend,
       eTransitionEnd,
       EventNameType_All,
       eTransitionEventClass)
+EVENT(transitioncancel,
+      eTransitionCancel,
+      EventNameType_All,
+      eTransitionEventClass)
 EVENT(animationstart,
       eAnimationStart,
       EventNameType_All,
       eAnimationEventClass)
 EVENT(animationend,
       eAnimationEnd,
       EventNameType_All,
       eAnimationEventClass)

--- a/widget/EventMessageList.h
+++ b/widget/EventMessageList.h
@@ -335,16 +335,17 @@ NS_EVENT_MESSAGE(eContentCommandScroll)
 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(eTransitionCancel)
 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)
 NS_EVENT_MESSAGE(eWebkitAnimationEnd)