@@ -28,7 +28,7 @@ import { atomFeed, catalogCsv, commitDatesFromLog, DATED_PATHS, homeJsonLd, site
 import { schemaErrors } from "./lib/schema-check.mjs";
 import { llmsFullTxt, llmsTxt } from "./lib/llms.mjs";
 import { deprecationReason, MarkdownError, renderBlocks, renderInline, splitEntryBody } from "./lib/markdown.mjs";
-import { checkPage } from "./lib/page-checks.mjs";
+import { checkPage, pageIds } from "./lib/page-checks.mjs";
 import { checkStacks } from "./checks.mjs";
 import { stackPageHtml } from "./lib/stack-page.mjs";
 import { freshnessLineHtml, freshnessPageHtml, freshnessStats, newEntriesLineHtml } from "./lib/freshness.mjs";
@@ -525,7 +525,21 @@ function checkPages(files) {
     if (produced.has(rel ? `${rel}/index.html` : "index.html")) return "dir";
     return null;
   };
-  return files.flatMap((f) => checkPage(fs.readFileSync(path.join(DIST, f), "utf8"), { file: f, kindOf }));
+  // A #fragment is checked against the ids of the page it points at.
+  const ids = new Map();
+  const idsOf = (rel) => {
+    if (!ids.has(rel)) {
+      let found = null;
+      try {
+        found = pageIds(fs.readFileSync(path.join(DIST, rel), "utf8"));
+      } catch {
+        // Not a readable page: kindOf has already vouched for it existing.
+      }
+      ids.set(rel, found);
+    }
+    return ids.get(rel);
+  };
+  return files.flatMap((f) => checkPage(fs.readFileSync(path.join(DIST, f), "utf8"), { file: f, kindOf, idsOf }));
 }
 
 function main() {
 
@@ -179,6 +179,16 @@ const okPage = `<html><head><title>T</title><link rel="canonical" href="https://
 const kinds = new Map([["entry/e2", "dir"], ["", "dir"]]);
 const opts = { file: "entry/e1/index.html", kindOf: (p) => kinds.get(p) || null };
 eq("clean page passes", checkPage(okPage, opts).length, 0);
+{
+  const withLinks = (links) => okPage.replace("<h1>A</h1>", `<h1 id="top">A</h1>${links}`);
+  const fopts = { ...opts, idsOf: (p) => (p === "entry/e2/index.html" ? new Set(["preview-images"]) : null) };
+  eq("fragment: a same-page id passes", checkPage(withLinks('<a href="#top">t</a><a href="#">x</a>'), fopts).length, 0);
+  has("fragment: a missing same-page id fails", checkPage(withLinks('<a href="#nowhere">t</a>'), fopts).join(), "link #nowhere names no id on this page");
+  eq("fragment: a cross-page id passes", checkPage(withLinks('<a href="../e2/#preview-images">p</a>'), fopts).length, 0);
+  has("fragment: a missing cross-page id fails", checkPage(withLinks('<a href="../e2/#no-such-anchor">p</a>'), fopts).join(), "names no id on entry/e2/index.html");
+  eq("fragment: an encoded id is decoded", checkPage(withLinks('<a href="#t%6Fp">t</a>'), fopts).length, 0);
+  eq("fragment: unknown target ids are not guessed", checkPage(withLinks('<a href="../../#search">s</a>'), fopts).length, 0);
+}
 has("leaked bold", checkPage(okPage.replace("<h1>A</h1>", "<h1>A</h1>**b**"), opts).join(), "double asterisk");
 has("leaked link", checkPage(okPage.replace("<h1>A</h1>", "<h1>A</h1>[a](b)"), opts).join(), "markdown link");
 has("leaked backtick", checkPage(okPage.replace("<h1>A</h1>", "<h1>A</h1>`x`"), opts).join(), "backtick");
 
@@ -10,7 +10,31 @@ const LEAKS = [
   [/`/, "a backtick"],
 ];
 
-export function checkPage(html, { file, kindOf }) {
+/** Every id="..." in a page, unescaped. */
+export function pageIds(html) {
+  return new Set([...String(html).matchAll(/\sid="([^"]+)"/g)].map((m) => unescape(m[1])));
+}
+
+/** The fragment of a link, decoded, or null when it has none worth checking ("#" alone, a text fragment). */
+function fragmentOf(href) {
+  const i = href.indexOf("#");
+  if (i === -1) return null;
+  const raw = href.slice(i + 1);
+  if (!raw || raw.startsWith(":~:")) return null;
+  try {
+    return decodeURIComponent(unescape(raw));
+  } catch {
+    return unescape(raw);
+  }
+}
+
+/**
+ * `kindOf(rel)` says whether a dist path is a "file", a "dir" (with an
+ * index.html) or missing. `idsOf(rel)`, when given, returns the ids of the
+ * built page at that dist path (or null when it cannot tell), so a link's
+ * #fragment is checked against the page it points at.
+ */
+export function checkPage(html, { file, kindOf, idsOf = null }) {
   const errors = [];
   const text = html
     .replace(/<script[\s\S]*?<\/script>/g, " ")
@@ -30,9 +54,16 @@ export function checkPage(html, { file, kindOf }) {
   const description = unescape(html.match(/<meta name="description" content="([^"]*)"/)?.[1] || "");
   if (description.length > SEO_DESCRIPTION_MAX) errors.push(`${file}: description is ${description.length} characters, over ${SEO_DESCRIPTION_MAX}`);
   const dir = path.posix.dirname(file);
+  let ownIds = null;
   for (const m of html.matchAll(/\s(?:href|src)="([^"]+)"/g)) {
     const href = m[1];
-    if (EXTERNAL_SCHEME_RE.test(href) || href.startsWith("#")) continue;
+    if (EXTERNAL_SCHEME_RE.test(href)) continue;
+    if (href.startsWith("#")) {
+      const frag = fragmentOf(href);
+      ownIds ??= pageIds(html);
+      if (frag !== null && !ownIds.has(frag)) errors.push(`${file}: link ${href} names no id on this page`);
+      continue;
+    }
     const scheme = href.match(ANY_SCHEME_RE);
     if (scheme) {
       errors.push(`${file}: link ${href} uses the unsupported scheme "${scheme[1].toLowerCase()}:"`);
@@ -45,7 +76,17 @@ export function checkPage(html, { file, kindOf }) {
       errors.push(`${file}: link ${href} leaves the site`);
       continue;
     }
-    if (!kindOf(target)) errors.push(`${file}: link ${href} does not resolve`);
+    const kind = kindOf(target);
+    if (!kind) {
+      errors.push(`${file}: link ${href} does not resolve`);
+      continue;
+    }
+    const frag = fragmentOf(href);
+    if (frag !== null && idsOf) {
+      const page = kind === "dir" ? (target ? `${target}/index.html` : "index.html") : target;
+      const ids = page.endsWith(".html") ? idsOf(page) : null;
+      if (ids && !ids.has(frag)) errors.push(`${file}: link ${href} names no id on ${page}`);
+    }
   }
   return errors;
 }