[css-device-adapt][cssom] Missing VIEWPORT_RULE definition

I assume @viewport will introduce its own VIEWPORT_RULE type to the CSSRule interface [1].

At what stage does that happen i.e. when does a new rule type get assigned in the CSSWG 0-1000 range?

Given that css-device-adapt is a WD, I assume implementations would currently pick a value outside the range and expose a VENDOR_VIEWPORT_RULE constant. 



[1] http://dev.w3.org/csswg/cssom/#the-cssrule-interface