You know that thing where you find a really encouraging specification, and you go and write a bunch of code against it, only to find that your browser doesn’t implement it, no browser implements it, nobody’s written a polyfill for it, and the editors aren’t even really sure it’s a good idea? (Or, on the flip side, that it’s an ivory-tower pipe dream from the W3C circa 2003, nobody has even glanced at it in over a decade, and it never even could have been implemented, like some legacy CSS and Semantic Web specs?)
This would be for that, and also for potential editors: basically a live embed inline for each clause of a specification that takes all the best parts of http://caniuse.com (specifically its automated tests), https://www.chromestatus.com/features (specifically its tracking of vendor signals and consensus), and MDN compatibility tables (specifically the in-depth documentation of quirks across versions).
This could also link to notes on discussions of each section, and stuff like specs or non-standard practices that have superseded it.
See also:
http://specifiction.org/t/beyond-specifiction-giving-web-developers-a-voice/602