Dynamic investigation of ellipses inscribed in a rectangle

Described is a dynamic investigation of an ellipse inscribed in a rectangle, with a view to properties conserved while making changes. We find mathematically the locus from which an ellipse is viewed at an angle of 90°, as well as the locus of the foci of an ellipse rotated in a rectangle that inscribes it. Would also study the case where a given ellipse is inscribed by a parallelogram, the lengths of whose sides can be changed.