Changing the presentation often involves more than changing the CSS alone. For example, you might have a table of data that you want to change into a bootstrap .row/.col structure. The data itself has not changed, only the presentation, and this required a change of HTML.

The CSS always needs enough hooks in the HTML to do it's work, without the right hooks some things are impossible with CSS alone (or would be unnecessarily obscure/tricky/brittle).

HTML isn't a general data format in the way JSON is.