Not quite sure how this would work, but I'm imagining a browser setting or flag that users could set to prefer low bandwidth media consumption. Something like "prefers-reduced-motion", but "prefers-low-bandwidth"
In the case of images, it might choose the lowest image in a `srcset`, maybe, regardless of the browser window / agent's parameters. In streaming media, it could prompt or analyze the option to download vs stream in instances where downloading would be advantageous for minimizing bandwidth / energy consumption.