In the specific case of html and css I agree that it rarely works. There are a few examples like subreddit css, but in general html does to much layout to meaningfully separate content from layout. Xml and XSLT is a much better separation, with Xml containing pure content and Xslt transforming that into XHTML with CSS for presentation. But that never got much adoption.

In other domains separating content from presentation works well. But in web development the incentives don't align with that idea.

It’s really not nonsense. Make a text file with line breaks to force a line length, and then make a non-trivial edit to the content. You’ll have to move all the line-breaks too. It’s awful.

If you’d instead left presentation to the display tool, you’d only need to change the content.

The HTML presentation/content argument is this scaled up.

The separation of content from presentation is one of the primary purposes of MVC frameworks. The model (content) is separated from the view (presentation) by an abstraction layer (controller).