The transitionstart event is fired when a CSS transition has actually started, i.e., after any transition-delay has ended.

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: transitionstart.

This code adds a listener to the transitionstart event:

window.addEventListener('transitionstart', () => {
  console.log('Started transitioning');
});

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

window.ontransitionrun = () => {
  console.log('Started transitioning');
};

See a live example of this event.

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