Only supported by chrome for now

1. A Save-Data header is sent on each HTTP request. This allows dynamic backends to change the HTML returned.
2. The NetworkInformation.saveData JavaScript API. This allows client-side JavaScript to check this and act accordingly.
3. The upcoming prefers-reduced-data media query, which allows CSS to set different options depending on this setting. This is available behind a flag in Chrome, but not yet on by default while it finishes standardization.