The decoupling of CSS and HTML is a lie.

HTML describes layout, not just content.

Your overall structure is going to be vastly different whether you use flex or grid. And if you don’t want to write overly clever CSS you’re wrapping your dt/dd pairs in divs or you put dummy elements into your sections to control where the anchor links scroll to etc. And don’t get me started on non-hierarchical layout relationships.

HTML is not flexible enough and CSS is not expressive enough for this theoretical separation to hold in practice. You would need a translation layer between pure content and layout for this to be true (like XSLT).