The styling should simply serve to improve the content. The content should generally stand on its own without any styling.
In semantic CSS the goal is to write your HTML with as little regard to presentation as possible. Your HTML is simply structuring human readable data. At this step you give it no thought of appearance.
Styling that structured data comes second. You write the CSS to make the structured data look nice, and ideally do not touch the HTML in the process, at all. The styling is but an affectation.
This fell out of favor because people cared more about looking flashy than quality content, they didn't want to put any thought into developing the actual page and wanted to start with the design. It's a symptom of the overall disease of the modern web.
This is more achievable with modern CSS than ever before and I pray for it to make a comeback.