User Agent Properties

CC-ing the Publishing Working Group on this subject at the request of
Ivan (it was originally sent to the EPUB CG). 

Would it be imaginable the Publishing Working Group takes part in the
User Agent Properties discussion [1]? 

It is my understanding that it could impact personalization and, more
generally, a11y (OS preferences, UA user settings, etc.). 

Now, my concern is that some "Reading Solutions" might not be able to
set them, especially EPUB cloud readers for instance. 

Since Apple has partly implemented the proposal already (for iPhone X's
safe area insets), it would be preferable to check what could fit in
there ASAP in my opinion. 

On a related note, we'll eventually have to deal with this in EPUB
(since it is now using the W3C CSS snapshot) and I really can't tell if
use of those UA properties once they are available to authors won't
break some existing Reading Systems' user settings at first--all I know
is that Readium CSS would probably need an update. 

Jiminy 

Links:
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[1] https://github.com/w3c/csswg-drafts/issues/1693