Warn if link-defaults refers to W3C/WHATWG docs

[saschanaz]

Specs shouldn't use non-exported terms this way. Maybe it would help if bikeshed emit some warning (or even just some info-level log) that suggests filing an issue on the referenced spec.

[tabatkins]

I'm not sure what you mean by this. Can you elaborate on what you think is currently being done wrong?

[mkruisselbrink]

I think this is a similar request as #1792?

[tabatkins]

If so, this is the other direction from #1792.

[saschanaz]

Oh, didn't know #1792 exists. IMO it makes more sense to warn the one who references rather than the one who is being referenced, but either way may work.

One thing I wonder is whether warnings would be too destructive as one may pass --die-on everything on CI. Probably fine given that both spec-factory and spec-prod don't use such configuration by default.