If you look at other document writing system such as LaTeX. You will see that there are Sections, SubSections etc.
The link to the CSS simply told the browser how these should be presented. LaTeX has something similar as you can create a document class and Word has different styles you can import.
Having a concept of a document structure with a set of styles attached which you can change easily is a good idea.
The HTML document should make sense without a style sheet and if you write/generate semantic markup it does work properly.