A zettel representation that is a s-expression (also known as symbolic expression).
It is (relatively) easy to parse and contain all relevant information of a zettel, metadata and content. For example, take a look at the Sz encoding of this page, which is available via the Info sub-page of this zettel:
/z/00001012920516?enc=sz&part=zettel,
/z/00001012920516?enc=sz&part=meta,
/z/00001012920516?enc=sz&part=content.
Some zettel describe the Sz encoding in a more detailed way.
If transferred via HTTP, the content type will be text/plain.