Common Rel Types

[cholmes]

Each of our specs lists a number of rel types, and many of them are the same. We should have a single list that can be referenced with all the common rel types. Each spec may want to emphasize some as especially useful.

This came up in #884, and when we make the common list we should be sure to capture the ones it added.

[cholmes]

@m-mohr and I discussed on a call. The rel types are subtly different in some of their uses (like an item does not use 'child'), so making one general table is more challenging. But we may slim things down in the core spec and move some of the rel types to best practices.