Found at “Making the web better. With blocks! – Joel on Software” on 2022-01-28 16:01:16.
Blocks can be highly structured, that is, they can have types. That means that they magically become machine-readable without screen scraping. For example, if you want to create an event block to represent an event on a calendar, you will be able to specify a schema that describes the event data type in a standard way. That way tools like calendars can instantly parse and understand web pages that contain your event block, reliably.
This reminds me of WCM where the dream was essentially the same.