A head-mounted device may include a main housing portion and displays in the main housing portion that provide images to eye boxes. A light seal may be coupled to the main housing portion and may at least partially surround the eye boxes. The head-mounted device may include sweat mitigation structures such as moisture-guiding channels in the light seal to guide sweat and other moisture away from the eye boxes. The moisture-guiding channels may be formed from grooves or moisture-wicking fabric. Control circuitry may adjust a fan based on humidity, temperature, or fog detected by a sensor. The light seal may include a directional pattern of grooves through which moisture is guided away from the eye boxes. A moisture barrier layer may be interposed between an inner foam layer and an outer light-blocking fabric to prevent sweat from penetrating into the foam.