3.4.12 of EPUB Publications introduces the page-progression-direction 
attribute and further provides a note about other mechanisms, most 
notably the writing-mode and direction properties within HTML content 
documents, for indicating the page progression direction. 


http://idpf.org/epub/30/spec/epub30-publications.html#sec-spine-elem

However, nothing about the precedence rule is provided.  Thus, it 
is not clear which one wins and how the synthetic spread should 
be created

Unfortunately, there are no good solutions.

First, consider an EPUB publication for Japanese or Taiwanese manga 
such that each page is represented by a simple HTML document containing 
an image element and nothing else.  Since such HTML documents 
typically lack CSS, the default writing-mode and direction properties apply, 
and thus the left-to-right page progression is implied for these HTML 
documents.  However, note that this implied PPD is never used, since each HTML 
represents a single page.  The PPD attribute at the spine specifies 
right-to-left.  
The RS should exhibit the right-to-left behaviour and construct the synthetic 
spread by using the right page first.  In other words, the PPD at the 
spine should win. 

Second, consider a reflowable EPUB document representing a Japanese or 
Taiwanese novel in vertical writing or Hebrew/Arabic book.   HTML content 
documents in such an EPUB publication have CSS stylesheets and the implied 
PPD is right-to-left.  The RS should exhibit the right-to-left behaviour and 
construct the synthetic spread by using the right page first.  Even if the 
spine 
lacks the PPD attribute or specifies left-to-right, the PPD specified in 
content 
documents should win.

One could argue that the PPD at the spine should be right-to-left in the second 
case, and the PPD at the spine should always win.  However, some books in Asia 
have *both* right-to-left and left-to-right directions.  This may sound crazy, 
but 
is very natural when a book has two chapters: one (e.g., advise about education 
in general) in vertical writing and another (e.g., a collection of math 
exercises) 
in horizontal writing.  Thus, we cannot easily say that the PPD at the spine 
should win.

There were a lot of discussions about this topic in the EGLS Taipei meeting.  
In 
the end, everybody agreed that we need more experiences for handling 
combinations of right-to-left and lef-to-right, partly because the UI would 
be very confusing.