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Are you talking about mobile ? Sure there are less pixels but the size if my phone (i.e. the size of my pocket) is between 15 (laptop) and 25 times (desktop screen) smaller in area. My pocket will be smaller than my arms or my desk. So a different site presentation will be in order.

You can probably use UA Client Hints to do this; but it requires some custom-fu and doesn't work in all browsers.

You can do this with only media queries in CSS, most likely.

The disadvantage there is that sometimes you want to give the user a totally different site if their client is mobile. CSS queries are indiscriminate in that a smaller browser window may trigger the “mobile” css. Likewise, many tablets have similar screen sizes to some laptops, yet often you don’t want to present the same UI to a tablet and laptop.