Isn't this a trivial problem to solve that doesn't require introducing any new layout mechanisms?

Today it's usually implicitly designed for iphone, designed for 1080p, or ipad, and you have to guess, strong correlation with whatever device the designer uses in his personal life.

Today sites certainly can and some (many) so. But some (also many) definitely don't…

A lot are locked to a maximum width, which is OK enough as l……o……n……g lines of text are unpleasant to read, but only because browsers hack the meaning of dimension settings to make text zoom work consistently.

A lot also have an effective minimum width (even if they use responsive styling to move/ minimise/hide side decoration before a certain point) that is not always convenient. Try browsing with a thin window so you can have something in the other side of your screen. Some assume no one on desktop will ever have a browser window less wide than 1280 pixels (or equivalent on a zoomed higher res screen) - not the case on my 1080p portrait screen and I sometimes want things thinner than 1280 on my 2560x1440 screen. You could say I'm just odd and they can't cater for everyone, but 1080 or a bit less wide is hardly miles away from many devices physical layout so if a design can't display nice in that can it really call itself "responsive" (I suspect any such design would fail on many mobile devices too - 1080px effective width is rather common there, as are smaller widths).