should this include TAG documents like the EWP, Web Platform Design Principles, etc.?

[hober]

[dontcallmedom]

The main value one gets from adding non-spec documents to the web-specs or browser-spec lists is that they get crawled/indexed by reffy with data published in webref, which among other things make the marked up definitions available for re-use in ReSpec and listed in WebDex.

So the extent documents benefit from that, it probably makes sense to add them; conversely, if they don't, while they would absolutely be in scope (and a related PR would be merged), this may not bring much value to anyone at this stage.