[CSSWG][css-content] FPWD of CSS Generated Content L3

Okay, so it's *technically* not a FPWD, but since it's a near-complete
rewrite of the 2003 draft, I'm announcing it as one, since it warrants
that level of review. :)

The CSS WG has published an updated Working Draft of the
CSS Generated and Replaced Content Module Level 3:

     https://www.w3.org/TR/css-content-3/

This is the first draft of a complete rewrite of this module, and is
very rough. Major new functionality over Level 2 includes imports from
the GCPM module such as:
   * leaders
   * target-counters
   * named strings
   * bookmark control
plus a handy new syntax for alternate text:
   content: url(sparkle.png) / "New!"

We expect future work on this module to be pinning down the actual
syntax and behavior of all the exciting new features imported from
GCPM. For those who want to toy with implementations, most CSS->PDF
renderers already support a significant subset...

Significant changes since 2003 are listed at:
   https://www.w3.org/TR/2016/WD-css-content-3-20160602/

Please review the draft, and send any comments to the archived mailing
list <[email protected]>, prefixed with [css-content] (as I did on this
message) or (preferably) file them in the CSSWG GitHub repository at
   https://github.com/w3c/csswg-drafts/issues

For the CSS WG,
~fantasai