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Spoiler alert: There is no support for it yet. But it is defined in the Media Queries Level 5 spec that includes other recent, but more familiar user preference features, like prefers-color-scheme and prefers-reduced-motion.

The Polypane blog goes into incredible depth on prefers-reduced-data, especially for something that we have yet to see in the wild. That’s actually what makes the Polypane team an ideal voice on the subject. It’s product is a browser that is able to emulate the feature behind a Chromium flag.

At the same time, it’s worth noting that the spec calls out two significant potential issues with this feature in its current state:

• It may be an undesired source of fingerprinting, with a bias towards low income with limited data. 
• This feature is an early draft, and the CSS-WG does not consider it ready for shipping in production.

But that’s not to say we can’t start getting acquainted with it. Here’s how it works:

@media (prefers-reduced-data: reduced) { /* Stuff for reduced data preferences */
} @media (prefers-reduced-data: no-preference) { /* Stuff for no data preferences */
}

What I appreciate from this post is the wealth of possible use cases it lists. Allow me to summarize:

• Conditionally load fonts. As in, make a @font-face declaration then call the font on the body, once for users with no-preference to get the custom font, and again for users with reduced to get a lighter stack.
• Background images. Have you ever used a giant splash image as the background for some full-width hero component? Maybe that can be served just to folks who have no-preference while those with reduced get either a smaller variation or no image at all.
• Smaller images in HTML. This is clever because remember that we have the media attribute on the <source> element. So, we can instruct browsers to use certain images when working with <picture>, a la: <source srcset="small.jpg" media="(prefers-reduced-data: reduce)" />.
• Conditionally preload and autoplay videos. Just as we can work with this feature in HTML, we can use it in JavaScript, too, by using window.matchMedia('(prefers-reduced-data: no-preference)').matches to set autoplay and preload attributes on a video based on data preferences.
• Ditch infinite scrolling. I’d generally ditch this pattern in the first place, but we can certainly disable it so users who prefer reduced data aren’t force-fed more content (and thus, more data) just by reaching the end of a page.

That’s not a definitive list of ideas, of course! We talk all the time about serving the right assets to the right people at the right time and this media feature is a great tool to help us do that in certain contexts. Think also of:

• Providing low-res versions of downloaded assets (e.g. PDFs)
• Linking to certain sites or pages that have heavy experiences
• Conditionally loading entire scripts, stylesheets, or libraries based on preference
• Probably a gazillion other and more clever things…

And this advice is golden:

Like prefers-reduced-motion, it’s good to think of the prefers-reduced-data: reduce option as the default option: people get the lean, fast experience, and only when they indicate no-preference, we send them more data. That way, older browser that don’t support the media query get the lean experience by default.

Yep. The same sort of idea as “mobile-first” responsive design: start by designing for reduced data and then enhance as we scale up.

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