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

The difference is transitionstart and transitionrun is that transitionrun fires when the transition is created (i.e. at the start of any delay) and transitionstart fires when the actual animation has begun (i.e. at the end of any delay).

The original target for this event is the Element that had the transition applied. You can listen for this event on the Document 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:

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

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

document.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 Window targets: transitionstart