I actually wondered initially, even if CDL was legal, how are they going to enforce the actual Controlled part? What's the standard for that?
Is simply not having a download button sufficient? Do you need to prevent people from right-clicking and choose "Save Image"? Or do you need to run the book through a video stream encrypted with Widevine L3 so screenshots don't work? Or heck, why not Widevine L1 which requires hardware not found on Windows because L3 will always be exploitable?
Adobe Digital Editions DRM and watermarking appear to be popular. It's definitely breakable (e.g. with Calibre and DeDRM, this is even required if you use an obscure E-Reader which does not support it), but secure enough that the publishers are happy.
Is simply not having a download button sufficient? Do you need to prevent people from right-clicking and choose "Save Image"? Or do you need to run the book through a video stream encrypted with Widevine L3 so screenshots don't work? Or heck, why not Widevine L1 which requires hardware not found on Windows because L3 will always be exploitable?