[css-priorities] Updated document on CSS priorities

Since the meeting yesterday I've continued to work on the priorities
document on GitHub [1].

I've done some triage by splitting the table of priorities into three
parts: First, features that just require more implementations, as the spec
is stable and they already exist in several browsers. Second, features that
require more spec work. Third, features that require some fundamental
design work before specs can be written (many aspects of pagination fall
into this category, I believe).

I've also continued to add more features from the Google Spreadsheet [2]. I
still have some work to do, especially for math, hyphenation, and
pagination.

* * *

I would appreciate feedback on the whole idea, as well as more specific
assistance in the various subject areas, especially CJK.


Thanks,

Dave

[1] http://w3c.github.io/dpub-pagination/priorities.html
[2]
https://docs.google.com/spreadsheets/d/15IsDMPwSXx197Iqe4I9xh7K8anmJ5c0-OFEG7w0LHYM/edit?usp=sharing