The ontransitionrun, ontransitionstart, and ontransitioncancel event handler attributes allow developers to add event listeners for 'transitionrun', 'transitionstart', and ‘transitioncancel' events on elements, Document objects, and Window objects. An example of how this might be used: target.ontransitioncancel = () => { console.log('transition was cancelled!'); }
These features are part of the stable css-transitions-1 spec, and have shipped in other browsers a long time ago.
Docs: https://output.jsbin.com/siyenag/quiet
Explainers: N/A - spec has been stable for a long time