code: function observeResize (el, cb) {
    assert.ok(isDom(el), 'observe-resize: el should be a valid DOM element')
    assert.equal(typeof cb, 'function', 'observe-resize: cb should be type function')
  
    // Make this function a noop in non-browser environments
    if (typeof window !== 'object') return
  
    var called = false
    var frame = document.createElement('iframe')
    frame.setAttribute('class', '__observe-resize__')
    el.appendChild(frame)
  
    assert.ok(frame.contentWindow, 'observe-resize: no contentWindow detected - cannot start observing')
    frame.contentWindow.onresize = handleResize
  
    return function stopObserving () {
      if (frame.parentNode) frame.parentNode.removeChild(frame)
    }
  
    function handleResize () {
      if (called) return
      called = true
      window.requestAnimationFrame(function () {
        called = false
        cb(el)
      })
    }
  }
docs: Trigger a callback when an element is resized