The transitionrun event is fired when a CSS transition is first created, i.e. before any transition-delay has begun.

The original target for this event is the Element that had the transition applied. You can listen for this event on the Window interface to handle it in the capture or bubbling phases. For full details on this event please see the page on HTMLElement: transitionrun.

This code adds a listener to the transitionrun event:

window.addEventListener('transitionrun', () => {
  console.log('Transition is running but hasn't necessarily started transitioning yet');
});

The same, but using the ontransitionrun property instead of addEventListener():

window.ontransitionrun = () => {
  console.log('Transition started running');
};

See a live example of this event.

• The GlobalEventHandlers.ontransitionrun event handler
• The TransitionEvent interface
• CSS properties: transition, transition-delay, transition-duration, transition-property, transition-timing-function
• Related events: transitionend, transitionstart, transitioncancel
• This event on HTMLElement targets: transitionrun
• This event on Document targets: transitionrun