The point of a footer is that it is at the bottom of the canvas regardless of the size of other content. If it is not at the bottom of the canvas, then you get

The point of a footer is that it is at the bottom of the canvas regardless of the size of other content. If it is not at the bottom of the canvas, then you get something else at the bottom - like a colored box of unknown dimensions. Which means the footer is not the footer anymore.