An electronic device has pixels that form an active area of a display for displaying images for a user. A layer of black ink or other opaque material may be formed on an inner surface of a display cover layer in an inactive area of the display that does not overlap pixels. The housing may have sidewalls such as a rear housing wall that faces away from the display. Ambient light sensor windows may be formed from tapered holes or other holes. The tapered holes may be formed in the opaque material on the display cover layer, may be formed in a rear housing wall or other hosing structure, or may be formed in other portions of the electronic device. Non-tapered holes may also form windows. Tapered holes may have sidewalls with portions that run parallel to their longitudinal axes and portions that are angled relative to their longitudinal axes.