transition-to-auto From http://n12v.com/css-transition-to-from-auto/

new_gist_file

var prevWidth = element.style.width
element.style.width = 'auto'
var endWidth = getComputedStyle(element).width
element.style.width = prevWidth
element.offsetWidth // force repaint
element.style.transition = 'width .5s ease-in-out'
element.style.width = endWidth
element.addEventListener('transitionend', function transitionEnd(event) {
	if (event.propertyName == 'width') {
		element.style.transition = ''
		element.style.width = 'auto'
		element.removeEventListener('transitionend', transitionEnd, false)
	}
}, false)