The problem here is backwards compatiblity. As your HTML file evolves, you will want to restructure it. CSS only takes you that far, at some point you need to a

The problem here is backwards compatiblity. As your HTML file evolves, you will want to restructure it. CSS only takes you that far, at some point you need to adjust your HTML structure. And then you have the problem that most clients will break with the new HTML structure. In contrast, a separately designed JSON or XML structure (if you follow some simply sanity rules) can be kept backwards compatible for a long time.