Publishing a web feed: RSS 2.0, Atom or JSON Feed, and the details readers depend on

Goal

Give readers, aggregators and agents a machine-readable list of new content that they can poll cheaply and deduplicate reliably.

Prerequisites

A canonical URL, a stable identifier and a publication timestamp for every item, and server-side templating for the feed document.

Steps

1. Choose a format. Atom (RFC 4287) is the strictest: feed and entry must each contain exactly one id, title and updated; dates follow RFC 3339; content declares its type (text, html, xhtml). RSS 2.0 requires the channel elements title, link and description; all item elements are optional but at least title or description must be present; dates follow RFC 822 (Sat, 07 Sep 2002 00:00:01 GMT); guid identifies an item and its isPermaLink attribute defaults to true. JSON Feed 1.1 is the same model in JSON: a version URL, title, and items with a required id and content_html or content_text, served as application/feed+json. Atom or JSON Feed suits new sites; add RSS only if a consumer asks for it.
2. Use stable IDs: the item's canonical URL or a tag: URI that never changes, even when the title or slug does. Changing IDs makes every reader show old items as new.
3. Put full content in the feed (Atom content, RSS description with entity-encoded HTML, JSON content_html), sanitised like any HTML you serve, with absolute URLs for links and images.
4. Serve the right media type (application/atom+xml, application/rss+xml, application/feed+json), support conditional GET with ETag or Last-Modified as the JSON Feed specification recommends for publishers, and limit the document to the newest items with pagination for the archive.
5. Advertise the feed with <link rel="alternate" type="application/atom+xml" href="/feed.xml"> in the HTML head and a visible link on the page.
6. Validate with a feed validator and by subscribing in two readers; edit an item and check that it updates in place rather than duplicating.

Expected result

Readers see each item once, with correct dates and working links, and pollers receive 304 responses between changes.

Limits and test basis

Format facts come from the cited specifications; reader behaviour differs by product and is not measured here. Atom's updated means significant modification, not publication, and is easy to confuse with it; lenient RSS date parsing hides errors until a strict consumer appears.