The applet is supposed to suggest the following attractive fact (and its proof):

To see why this is so, extend AC to AF with CF = CD. Triangles ACE and BCF are equal and are obtained from each other with a rotation through 90°. This is then the angle between their corresponding elements. In particular, BF is perpendicular to AE and is therefore parallel DK and CL. This gives two lines AB and AC crossed by three parallel lines, DK, CL, BF. Since CD = CF, so also KL = BL.

References