Unless the definition of "non-proprietary" is really unorthodox/counterintuitive, this regulation basically requires making publications available in a DRM-free ePub format. A publisher could sabotage this by using a weird format and a free DRM, but that would be pretty obvious (like publishing in HyperCard).

Other than ePub, the living eBook formats, all are proprietary (PDF coming closest as an alternative); there are free legacy formats (XML, HTML, OEBPS, ODP...) and a few cult formats, but they're rarely found in the wild. Amazon's Kindle doesn't support ePub so ePub is often used as the source for conversion to an eReader-supported proprietary format.

The ePub format *does* support DRM, but the DRM schemes in use are proprietary and there's little (if any) inter-compatibilty. Amazon & Apple DRMs are restricted to their own markets, and Adobe's DRM would make it impossible to read an ePub on most devices (or convert it to Kindle). I'm sure it's possible to come up with a non-proprietary DRM for ePub, but it would probably make the document unreadable on any eReader.

/AFAIK