feat: add heading anchors for direct linking to article sections

Uses markdown-it-anchor to generate linkable IDs on h2-h4 headings.
Headings become clickable links with a subtle # indicator on hover.
Includes scroll-margin-top so anchored headings don't hide behind
the sticky header.

css/tailwind.css

@@ -468,6 +468,31 @@
     scroll-margin-top: 80px; /* Prevent header overlap when scrolling to anchors */
   }
 
+  /* Heading anchors — generated by markdown-it-anchor */
+  .prose h2[id],
+  .prose h3[id],
+  .prose h4[id] {
+    scroll-margin-top: 80px;
+  }
+  .prose :is(h2, h3, h4) > a.header-anchor {
+    color: inherit;
+    text-decoration: none;
+  }
+  .prose :is(h2, h3, h4) > a.header-anchor:hover {
+    text-decoration: underline;
+    text-decoration-color: var(--tw-prose-links, currentColor);
+    text-underline-offset: 4px;
+  }
+  .prose :is(h2, h3, h4) > a.header-anchor::after {
+    content: " #";
+    opacity: 0;
+    font-weight: normal;
+    transition: opacity 0.15s;
+  }
+  .prose :is(h2, h3, h4):hover > a.header-anchor::after {
+    opacity: 0.4;
+  }
+
   .post-list li {
     content-visibility: auto;
     contain-intrinsic-size: auto 200px;

eleventy.config.js

@@ -4,6 +4,7 @@ import embedEverything from "eleventy-plugin-embed-everything";
 import { eleventyImageTransformPlugin } from "@11ty/eleventy-img";
 import sitemap from "@quasibit/eleventy-plugin-sitemap";
 import markdownIt from "markdown-it";
+import markdownItAnchor from "markdown-it-anchor";
 import syntaxHighlight from "@11ty/eleventy-plugin-syntaxhighlight";
 import { minify } from "html-minifier-terser";
 import { createHash } from "crypto";
@@ -48,6 +49,11 @@ export default function (eleventyConfig) {
     linkify: true,  // Auto-convert URLs to clickable links
     typographer: true,
   });
+  md.use(markdownItAnchor, {
+    permalink: markdownItAnchor.permalink.headerLink(),
+    slugify: (s) => s.toLowerCase().replace(/[^\w\s-]/g, "").replace(/[\s_-]+/g, "-").replace(/^-+|-+$/g, ""),
+    level: [2, 3, 4],
+  });
   eleventyConfig.setLibrary("md", md);
 
   // Syntax highlighting for fenced code blocks (```lang)

package-lock.json

@@ -19,6 +19,7 @@
         "eleventy-plugin-embed-everything": "^1.21.0",
         "html-minifier-terser": "^7.0.0",
         "markdown-it": "^14.0.0",
+        "markdown-it-anchor": "^9.2.0",
         "pagefind": "^1.3.0",
         "rss-parser": "^3.13.0"
       },
@@ -1053,6 +1054,31 @@
         "tailwindcss": ">=3.0.0 || insiders || >=4.0.0-alpha.20 || >=4.0.0-beta.1"
       }
     },
+    "node_modules/@types/linkify-it": {
+      "version": "5.0.0",
+      "resolved": "https://registry.npmjs.org/@types/linkify-it/-/linkify-it-5.0.0.tgz",
+      "integrity": "sha512-sVDA58zAw4eWAffKOaQH5/5j3XeayukzDk+ewSsnv3p4yJEZHCCzMDiZM8e0OUrRvmpGZ85jf4yDHkHsgBNr9Q==",
+      "license": "MIT",
+      "peer": true
+    },
+    "node_modules/@types/markdown-it": {
+      "version": "14.1.2",
+      "resolved": "https://registry.npmjs.org/@types/markdown-it/-/markdown-it-14.1.2.tgz",
+      "integrity": "sha512-promo4eFwuiW+TfGxhi+0x3czqTYJkG8qB17ZUJiVF10Xm7NLVRSLUsfRTU/6h1e24VvRnXCx+hG7li58lkzog==",
+      "license": "MIT",
+      "peer": true,
+      "dependencies": {
+        "@types/linkify-it": "^5",
+        "@types/mdurl": "^2"
+      }
+    },
+    "node_modules/@types/mdurl": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/@types/mdurl/-/mdurl-2.0.0.tgz",
+      "integrity": "sha512-RGdgjQUZba5p6QEFAVx2OGb8rQDL/cPRG7GiedRzMcJ1tYnUANBncjbSB1NRGwbvjcPeikRABz2nshyPk1bhWg==",
+      "license": "MIT",
+      "peer": true
+    },
     "node_modules/@types/node": {
       "version": "14.18.63",
       "resolved": "https://registry.npmjs.org/@types/node/-/node-14.18.63.tgz",
@@ -2812,6 +2838,16 @@
         "markdown-it": "bin/markdown-it.mjs"
       }
     },
+    "node_modules/markdown-it-anchor": {
+      "version": "9.2.0",
+      "resolved": "https://registry.npmjs.org/markdown-it-anchor/-/markdown-it-anchor-9.2.0.tgz",
+      "integrity": "sha512-sa2ErMQ6kKOA4l31gLGYliFQrMKkqSO0ZJgGhDHKijPf0pNFM9vghjAh3gn26pS4JDRs7Iwa9S36gxm3vgZTzg==",
+      "license": "Unlicense",
+      "peerDependencies": {
+        "@types/markdown-it": "*",
+        "markdown-it": "*"
+      }
+    },
     "node_modules/markdown-it/node_modules/entities": {
       "version": "4.5.0",
       "resolved": "https://registry.npmjs.org/entities/-/entities-4.5.0.tgz",

package.json

@@ -20,6 +20,7 @@
     "eleventy-plugin-embed-everything": "^1.21.0",
     "html-minifier-terser": "^7.0.0",
     "markdown-it": "^14.0.0",
+    "markdown-it-anchor": "^9.2.0",
     "pagefind": "^1.3.0",
     "rss-parser": "^3.13.0"
   },