The ornamental design for a visor for a cap or headband, as shown and described.
FIG. 1 is an elevated perspective view of a visor for a cap or headband showing my new design, the broken line showing a cap being for illustrative purposes only and forms no part of the claimed design;
FIG. 2 is a top plan view of the visor of FIG. 1;
FIG. 3 is a front plan view thereof; and,
FIG. 4 is a side view thereof, the undisclosed side being a mirror image.