feat(websub): advertise the hub on both feeds

public/.well-known/api-catalog

@@ -26,6 +26,12 @@
           "title": "Changelog feed"
         }
       ],
+      "hub": [
+        {
+          "href": "https://specification.website/websub",
+          "title": "WebSub hub for this site's feeds"
+        }
+      ],
       "sitemap": [
         {
           "href": "https://specification.website/sitemap-index.xml",

public/_headers

@@ -64,23 +64,19 @@
   Content-Type: text/javascript; charset=utf-8
   Cache-Control: public, max-age=600, must-revalidate
 
-# Feeds — WebSub discovery. HELD BACK UNTIL THE HUB IS LIVE: advertising
-# rel="hub" while /websub cannot accept subscriptions is the exact failure
-# /spec/foundations/websub/ warns about (an advertisement is not a delivery
-# mechanism), so these blocks are uncommented in the same commit that
-# provisions the D1 database — see the checklist in wrangler.toml.
+# Feeds — WebSub discovery.
 #
 # rel="hub" names the hub that pushes updates for this topic; rel="self" is the
 # topic's canonical URL, which is the identity subscribers key on. Subscribers
 # must check Link headers before embedded elements (WebSub §4), so this is the
 # authoritative copy and the atom:link elements inside each feed are the
 # fallback. Keep all three in sync with ALLOWED_TOPICS in functions/websub.ts.
-#
-# /rss.xml
-#   Link: </websub>; rel="hub", <https://specification.website/rss.xml>; rel="self"
-#
-# /changelog/rss.xml
-#   Link: </websub>; rel="hub", <https://specification.website/changelog/rss.xml>; rel="self"
+
+/rss.xml
+  Link: </websub>; rel="hub", <https://specification.website/rss.xml>; rel="self"
+
+/changelog/rss.xml
+  Link: </websub>; rel="hub", <https://specification.website/changelog/rss.xml>; rel="self"
 
 # Long cache for fingerprinted assets
 /_astro/*

src/content/spec/foundations/websub.md

@@ -7,7 +7,7 @@ status: optional
 order: 130
 appliesTo: [all]
 relatedSlugs: [feed-discovery, feed-hygiene, canonical-url, link-headers]
-updated: "2026-07-30T00:00:00.000Z"
+updated: "2026-07-31T00:00:00.000Z"
 sources:
   - title: "WebSub — W3C Recommendation, 2 June 2026"
     url: "https://www.w3.org/TR/websub/"
@@ -62,6 +62,8 @@ Or, inside an Atom feed:
 
 **4. Keep the feed correct anyway.** WebSub is additive. Subscribers that do not implement it keep polling, so [feed hygiene](/spec/foundations/feed-hygiene/) — stable GUIDs, valid markup, correct `Last-Modified` and `ETag` — still governs how well those clients behave. The push path carries the same feed body, so a malformed feed is malformed for everyone.
 
+**This site ships it.** Both [`/rss.xml`](/rss.xml) and [`/changelog/rss.xml`](/changelog/rss.xml) advertise `rel="hub"` and `rel="self"` on the response and inside the feed, pointing at a hub we run ourselves at `/websub`. It is deliberately a *self-hub*: `hub.topic` is allowlisted to those two feeds, so the endpoint cannot be pointed at arbitrary URLs or made to relay anyone else's content. An open hub is a public service with a different risk profile, and running one is not a prerequisite for the advice above — pointing at a hosted hub is the ordinary choice.
+
 ## Common mistakes
 
 - Advertising `rel="hub"` without pinging the hub on publish. Nothing is ever distributed, and nothing errors.

src/pages/changelog/rss.xml.ts

@@ -56,9 +56,9 @@ export async function GET(context: APIContext) {
     customData: [
       "<language>en-GB</language>",
       `<atom:link href="${feedUrl}" rel="self" type="application/rss+xml" />`,
-      // WebSub hub for this topic — held back until the hub is live, along with
-      // the Link header in public/_headers. See the checklist in wrangler.toml.
-      // `<atom:link href="${base}/websub" rel="hub" />`,
+      // WebSub hub for this topic. Fallback copy of the Link header in
+      // public/_headers — subscribers check headers first (WebSub §4).
+      `<atom:link href="${base}/websub" rel="hub" />`,
       `<lastBuildDate>${lastBuild.toUTCString()}</lastBuildDate>`,
       "<sy:updatePeriod>weekly</sy:updatePeriod>",
       "<sy:updateFrequency>1</sy:updateFrequency>",

src/pages/rss.xml.ts

@@ -34,9 +34,9 @@ export async function GET(context: APIContext) {
     customData: [
       "<language>en-GB</language>",
       `<atom:link href="${feedUrl}" rel="self" type="application/rss+xml" />`,
-      // WebSub hub for this topic — held back until the hub is live, along with
-      // the Link header in public/_headers. See the checklist in wrangler.toml.
-      // `<atom:link href="${new URL("/websub", siteUrl).toString()}" rel="hub" />`,
+      // WebSub hub for this topic. Fallback copy of the Link header in
+      // public/_headers — subscribers check headers first (WebSub §4).
+      `<atom:link href="${new URL("/websub", siteUrl).toString()}" rel="hub" />`,
       `<lastBuildDate>${lastBuild.toUTCString()}</lastBuildDate>`,
       "<sy:updatePeriod>weekly</sy:updatePeriod>",
       "<sy:updateFrequency>1</sy:updateFrequency>",