For some data types (URLs in particular, but also some external identifiers), it will be important to support property-specific formatting rules when displaying the human-readable label for a given value.

With this feature in place, the user could enter an alphanumeric string value for a property like Instagram username, and see a live link to the appropriate web page upon saving.

If formatting rules already exist for a particular property, the Wikibase format value API action will apply them propertly. We just need to update some API calls and related logic.