# The preview protocol

Source: https://capacms.com/docs/preview/protocol

The messages between the Capa editor and your page, for stacks other than Next.js.

Every message is `{ source, v: 1, type, ...fields }`. `source` is
`capa-admin` on the admin's messages and `capa` on the site's. The admin sends
`hello`, `highlight { entryId, field }` (`entryId: ""` clears), `outline { on }`
and `refresh`. The site answers `ready { path, entries }` after every hello and
every navigation, `select { entryId, field }` on a click, and `hover`.
`acceptMessage(event, allowedOrigins, parent)` is the site's filter, exported
for your own tests.

The site also sends `visible { entryId, field }` while the page scrolls (at
most every 150ms, and only when it changes): the tagged element at the centre
of the viewport, chosen by `pickCentred`. At the very top of a page, where a
heading can never reach the centre, it is the topmost visible element instead,
and at the very bottom the bottommost. The editor's "Follow the page" scrolls
the form to that field. An overlay older than 1.0.0-next.2 never sends it, and
the editor then does not follow.