Provided one can access sensor data fast enough, cross device tracking[0]. Display an ad that lights up the room in such a way that you can read it with the sensor. Communication across an airgap. If you have two devices with screens and said sensors in a dark room, they might be able to communicate by turning screens on and off.

Most speculatively, imaging the environment with compressive imaging[1]. One might be able to flash some patterns on the screen and look at light sensor output to take a picture.

Giving web browsers access to sensors on our devices is sort of scary.

[0] http://arstechnica.com/tech-policy/2015/11/beware-of-ads-tha... [1] http://arxiv.org/pdf/1305.7181.pdf