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.

Your use case sounds very much like an edge-case in websites. Of course highly customized animations and weird out-of-the-ordinary UI elements are going to be an extra pain in the ass. I have no doubt there are customer requests that simply can't be fulfilled with HTML and have to be done in WebGL. The fact is, most sites are "fairly basic" and only really need HTML, CSS, and a little javascript to accommodate the vast majority of use cases. I'm glad you're doing complicated things, but that doesn't change the reality of most web developers.

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.

Most SAAS tools aren't really used on mobile devices. Laptop or desktop is the expected use case. You're still dealing with an edge case if you need to have a "complex" SAAS tool that is mobile friendly.