@tek
#TLDR my brain-drippings generated by your toot, feel free to skip 
this is very interesting…
my instinctive response to this (as a long-ago web dev but casual keeper-upper with stuff) was, “don’t you get what you need from #UserAgent strings any more?” so I went (a short way) down a rabbit hole… 
despite recent changes by Google to restrict info leakage by Chrome user agents (

yay #privacy^), it seems like they still give plenty of info about devices that can be used to put them into at least buckets of “phone”, “tablet”, and “desktop”^^ (happy to be corrected, of course
)
do we really need websites tailored to the pixel rather than a few buckets based on approximate sizes, with layouts to match those sizes using percentage-based definitions for elements / containers?
is that not sufficient for decent responsive / adaptive design any more?
is the #UX *actually* significantly better using exact-pixel tailoring?
do sites see significantly higher bounce rates or lower sales conversion or whatever metric they care about if we approximate with well-thought-out layouts? 
[rant]what I mostly see these days (that I loathe) is web devs doing “mobile only” rather than “mobile first” layouts that are ridiculously large & shouty on desktops and provide almost no detail on anything ‘coz they sell to assume that no one will ever look at the site on a non-mobile device…
[/rant]
if a site offers me something that fits well enough into “mobile” or “desktop”^^^ (and *actually* allows me to switch between them if I explicitly request it to do so), AND gives me an on-page way to increase / decrease text size without altering other elements, I’m usually very happy (rant about WhyTF every mobile browser doesn’t bake per-site text size controls into their #UI left for another day
)
^ well, more like “privacy” since you can still pull all the details from headers or via JS so ¯\_(ツ)_/¯
^^ e.g. data from https://www.useragents.me/
^^^ bonus points for offering “tablet” as well