The saveData attribute is a boolean that indicates a client’s explicit opt-in for reduced data usage, due to high transfer costs, slow connection speeds, or other reasons. It's use allows JavaScript to make decisions that result in reduced data use, such as to load smaller images and video resources, or reduce buffering or predictive actions. This is expected to be enabled by default in Chrome 65.
Samples: https://googlechrome.github.io/samples/network-information/