It should be possible to build a DAT-capable browser for simple types of content (RST, MD, orgmode, etc); no HTML capability included. If you made it download media content instead of displaying it in-browser, it might even be simple enough to implement this without spending too much time chasing down security issues.
One could even go down to a slightly modified Gopher protocol. Just let publishers publish in whatever format they see fit (PDF, docx, html, ...) and let the users configure the viewer in their browsers.
