Block templates over the API, with a validator and a real preview

Two endpoints, both read-only

POST /templates/validate checks a document and returns every problem in it at once, where saving an invalid one gives you a single 400. If you generate documents programmatically, that is the endpoint to build the loop around: one call tells you everything to fix rather than one thing per round trip.

POST /templates/preview renders unsaved HTML or blocks through the same path a real send uses. Both need only templates:read, because neither writes anything, which means a key that is allowed to look at your templates is allowed to check them without also being allowed to change them.

Preview shows you what you did not write

Because it composes the way a send does, the preview includes the parts the pipeline adds: the unsubscribe block marketing mail gains, and the effect of the channel floor. Preview a transactional template as a campaign and the unsubscribe still appears, because a campaign is always marketing whatever the template says.

Neither of those is something a preview rendered in your own interface can tell you, which is the whole reason this is a server endpoint rather than a client-side render.

Null and omitted are different requests

One asymmetry to know about on update: a document of null ejects the template to raw HTML, while omitting the field leaves the authoring mode alone. A client that strips nulls before serialising can therefore never eject, and will look like it is ignoring you. If you mean to eject, send the null explicitly.