We've been using relative paths in the schemas to date as it made local development a little easier, but due to recent differences in how some validators work, we need to switch back to absolute URLs now that we're winding down the work. The problem is we never resolved on formal URLs for these.

For now, I've used the github pages address for the schemas: https://w3c.github.io/pub-manifest/schema/

While this seems to work fine, we should figure out if we want to put a W3C redirect on top of it, something like: https://www.w3.org/ns/pub-manifest/schema/*

Aside from not having github in the formal URL, it also gives us flexibility should we need to change locations in the future. (We'll also need a similar URL for the audiobooks schema, too.)

@iherman @marisademeglio and I have been discussing this privately while trying to iron out the validation problems the relative URLs caused, but what actual solution to use needs group approval.