Meta bug for tracking the upstreaming of CSS Device Adaptation initially started at bug 70370. It turned out that the implementation is quite big and would be difficult to review it in a single patch. Doing it incrementally and splitting into smaller patches might help.
*** Bug 70370 has been marked as a duplicate of this bug. ***
Mozilla has a bug for it: https://bugzilla.mozilla.org/show_bug.cgi?id=747754
CSS Device Adaptation is being removed from CSS: https://github.com/w3c/csswg-drafts/issues/4766