code: function getSize(element, prop) {
  	var style = getComputedStyle(element);
  	if (prop === 'width') {
  		return element.offsetWidth
  			- parseFloat(style.borderLeftWidth)
  			- parseFloat(style.paddingLeft)
  			- parseFloat(style.borderRightWidth)
  			- parseFloat(style.paddingRight);
  	}
  	else {
  		return element.offsetHeight
  			- parseFloat(style.borderTopWidth)
  			- parseFloat(style.paddingTop)
  			- parseFloat(style.borderBottomWidth)
  			- parseFloat(style.paddingBottom);
  	}
  }
docs: Get the computed content-box size
  
  @param  {Element} element
  @param  {string}  prop    `width` or `height`
  @return {number}