Re: [w3ctag/design-reviews] `prefers-reduced-data` CSS Media Query (Issue #705)

Hi Adam,

We discussed this again today, and in addition to our previous question about multi-stakeholder support, we had some additional concerns related to when this MQ is expected to change (which based on your previous comment seems to be left up to the implementation right now).

If it could change every time a user's connection speed changes, then this has the potential to be the worst of all worlds: **both** the low and high bandwidth versions of any CSS or HTML assets differentiated based on this MQ would be loaded, spending **more** of the user's bandwidth than if the MQ did not exist at all. We can see many well meaning authors doing things like:

```css
.foo { background: url("4k.jpg");

@media (prefers-reduced-data) {
 .foo { background: url("hd.jpg");
}
```

Imagine a user that starts off high bandwidth when the website is loading, then gets a reduction in network speed later. Not only have they actually downloaded the high bandwidth version of the website (and potentially paid a lot for it), but won't even get to enjoy it because **once the MQ flips, it will trigger the lowres version even if nothing is actually downloading when it changes!** Not to mention how jarring it will be to experience the change.

While it's easy to address that by caching the value of the MQ when the page loads and using a corresponding root class, in general in Web Platform features, the simple syntax should not be doing the potentially harmful thing.

If assets are loaded imperatively, this is also not ideal, because you could end up with a website with mixed assets, which may not even work well together. 

Lastly, there are also use cases where you *do* want the MQ value to reflect the most recent knowledge about the user's network speed or preference, e.g. in apps that load content continuously, so redefining this to remain fixed for the entire session duration would also leave a lot of use cases out. 😕 

-- 
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