See http://dev.w3.org/csswg/css3-animations/#dom-interfaces- for an example of this.
Be sure to update http://wiki.csswg.org/spec/cssom-constants with the value chosen.
We are still working on this, but we may be changing the syntax away from @region and use a pseudo-element syntax instead.
Work in progress at:
Added with https://dvcs.w3.org/hg/csswg/rev/2e04f2d16ef5
We've been coordinating CSSRule constants on this wiki page:
Regions is no longer one of the "proposals that have yet to be accepted by the working group," so I would have expected you to take a more conventional value, like 16.
Changed in http://dvcs.w3.org/hg/csswg/rev/c76a4ff42fc3