It isn't an edge case in the SAAS world, it's just that the majority of SAAS tools either don't implement mobile, or don't do it well. Every single SAAS company I have worked for in the last decade has had complex web apps that required very involved HTML and CSS work (and a dash of JS) to get them to work well on mobile. Additionally very few of the front-end or full-stack devs on those teams actually knew HTML/CSS well, they were all React/JS-first.
Sure the majority of websites on the web aren't SAAS tools, but the nuances of mobile vs desktop layout described in this article don't apply so much to such simple content pages.
Sure for a content-site template, blog, or something else fairly basic. This is not at all the case for things like art-directed super-custom designed sites like magazines, fashion brands, museums, etc, OR highly complex interactive SAAS web apps like the ones I work on. These have lots of unique considerations like complex animations, layered menus and controls, optimizing for change from mouse to touch interactions, handling the higher rate of people who use zoomed text on mobile devices, adapting to device safe areas in portrait and landscape, etc.
I am a designer and HTML/CSS coder so I am aware of the challenges from both directions.
Not really. No media queries, no percent widths or non-pixel units, no margins (everything is padding or gap), no viewport units, etc.
I'm a CSS person who works in Figma every day and Figma absolutely sucks for responsive design and handoff to devs.
I wish I could just design in Framer or Webflow but those tools while being Figma-like are entirely catered to content websites and not to product design.
Design tools should render with HTML and CSS, not make the mistake Figma made in creating their own rendering engine they have to painstakingly recreate CSS from scratch with.
Designers usually not the problem here. Most are well aware that rendering should change between different types of devices and displays, they use these website and devices too!
The problem usually is the resources and time available to implement such designs. Most of the time the devs simply don't have the time, desire, or even ability to implement more advanced responsive design, so designers will design what will actually get made.
Sure the majority of websites on the web aren't SAAS tools, but the nuances of mobile vs desktop layout described in this article don't apply so much to such simple content pages.