The DOM is more than the proposed content map, because it also creates structure. I think the proposed idea is to basically create a structure without any content, then apply the content into that structure.

The current split is CSS is presentation while HTML is content + structure. Unless I'm misunderstanding, this proposal would separate content and structure. I'm not sure where presentation would go; you could certainly make a case for it being grouped with either.

There's also plenty of questions around what's content and what's structure and what's presentation. For instance, a section heading is potentially all three. I personally would probably go for grouping presentation with the content, and call structure "layout" to disambiguate it. A section heading is maybe structure, but it's definitely not layout.