It seems like the "FART" problem is sort-of driving it, but the second URL contains a rough solution to it that doesn't require the (IMO crazy) idea in the main

It seems like the "FART" problem is sort-of driving it, but the second URL contains a rough solution to it that doesn't require the (IMO crazy) idea in the main article. I think the second linked article required an "edge function" to add the data- attribute to the HTML delivered to the client, and I think TFA's author doesn't like the need for that (i.e., wants to deliver a static document), but I think you could do it with some synchronous Javascript; the fallback for clients not wanting JS would be CSS media queries that just obeyed the UA's preferences… which seems like exactly what one should want…? (So, I'm also missing a convincing "why", here.)