To reply to you both - I'm specifically talking about the web. I have no experience building apps, so can't comment there. But for the web, I'm absolutely happy to argue it's not that much more effort.
Whether it's the web or an "app," it should work sensibly at any reasonable resolution. It doesn't have to look good, or look like the pixel-perfect, happy-path Photoshop mocks the UX designer made, but it should function. Desktop developers have been doing at least an OK job of this for decades.
Yet here we are, where it's a struggle to find an app that even sensibly supports landscape mode, or supports a user-configured font size.
Yet here we are, where it's a struggle to find an app that even sensibly supports landscape mode, or supports a user-configured font size.