[css-values-3][spec-rec] Publish CSS Values & Units 3 as a REC

From @atanassov's last email, given having something public to point at about what blocks publishing it would be nice:

• DOC - https://drafts.csswg.org/css-values-3/issues-cr-2016
• Looks like only one issue?

AIUI the blockers status are:

• Write as DOC for the 2019 CR
• #4840 (drop the at-risk multi-arg attr())
• Check references v. the normative reference policy; currently non-REC normative refs:
  • [CSS-CASCADE-4]
  • [CSS-COLOR-4]
  • [CSS-COUNTER-STYLES-3]
  • [CSS-DISPLAY-3]
  • [CSS-IMAGES-4]
  • [CSS-OVERFLOW-3]
  • [CSS-SIZING-3]
  • [CSS-UI-4]
  • [CSS-VALUES-4] (note this is a normative citation from level 3 to level 4!)
  • [CSS-WRITING-MODES-4]
  • [CSS3-BACKGROUND]
  • [CSS3-IMAGES]
  • [CSS3CASCADE]
  • [CSS3PAGE]
  • [CSS3SYN]
  • [CSSOM]
  • [MEDIAQ]
  • [SELECTORS-4] (note we also cite [SELECTORS-3] which is a REC)
• Check test suite status and write an implementation report

Start with the CSS Values & Units 3 CR issues document and the listed publication blockers. Check the normative reference policy against each non-REC reference, then review the test suite status and implementation-report requirements. Done means the 2019 CR documentation, references, at-risk item, and implementation report are all ready for REC publication.