Aaron Parecki

We wanted to avoid needing to spell out diffing mechanisms for every conceivable format in the core spec.

The trick around this to technically remain spec compliant, while also not surprising consumers of the WebSub API, is to say that your WebSub-enabled URL returns only the latest item. That way you can send just the one <article> or whatever, and it still counts as "sending the full contents of the URL". In practice, it's fine to do this as long as the page you're sending has a concept of multiple items in a list, and if your consumers are ready to handle arbitrary numbers of items.