I'm guessing the parent means using top, left, bottom and right offsets instead of transforms to support the likes of IE8.

The point of FLIP, RAIL and using transforms is to reliably achieve 60FPS by avoiding repaints whenever possible and giving the browser as little work to do as possible.

Adjusting width, absolute position offets and so on, can result in frames being dropped even in newer browsers on good hardware.

I personally treat animations as a luxury. If the user's browser doesn't support transforms (IE9 required for 2D, IE10/11 required for 3D) I just keep the content displayed in a clear and normal way.

Not sure I understand - changing absolute position also won't reflow parents/siblings, same as transform.