How to detect CSS3 resize events

Resizing is like a style change. As such it can be observed with a MutationObserver. The more specific ResizeObserver is probably even better:

let observer = new ResizeObserver(function(mutations) {
  console.log('mutations:', mutations);
});

let child = document.querySelector('textarea');
observer.observe(child, { attributes: true });

<textarea></textarea>

Listen to DOMAttrModified events. Got the idea from this answer, this jsFiddle appears to work in Firefox 8 (if you open the console).