diff --git a/css/tailwind.css b/css/tailwind.css
index 1bbafb7..1f027d7 100644
--- a/css/tailwind.css
+++ b/css/tailwind.css
@@ -1155,3 +1155,65 @@ body[data-indiekit-auth="true"] .share-post-btn:hover {
     background-color: rgb(142 192 124 / 0.1);
   }
 }
+
+/* ── Sidenotes ───────────────────────────────────────────────────────────── */
+/* Narrow: hide sidenotes; footnote <section> is visible by default          */
+
+.sidenote {
+  display: none;
+}
+
+.sidenote-host {
+  display: inline;
+}
+
+.footnote-ref-num {
+  font-size: 0.75em;
+  vertical-align: super;
+  line-height: 0;
+  font-weight: 600;
+  color: #458588; /* accent-400 */
+  font-variant-numeric: tabular-nums;
+}
+
+/* Wide (xl+): left gutter + floating sidenotes */
+@media (min-width: 1280px) {
+  /* Cancel the overflow:clip BFC so the float can escape the content box
+     into the padding gutter. The !important overrides Tailwind prose resets. */
+  article.has-sidenotes .e-content {
+    overflow-x: visible !important;
+    padding-left: 210px !important;
+  }
+
+  article.has-sidenotes .sidenote {
+    display: block;
+    float: left;
+    clear: left;
+    width: 190px;
+    margin-left: -210px; /* pull into the 210px padding gutter */
+    margin-top: 0.15em;  /* subtle optical alignment with line */
+    font-size: 0.8rem;
+    font-style: italic;
+    line-height: 1.5;
+    color: #7c6f64; /* surface-500 */
+  }
+
+  .dark article.has-sidenotes .sidenote {
+    color: #a89984; /* surface-400 */
+  }
+
+  article.has-sidenotes .sidenote-number {
+    display: block;
+    font-size: 0.7rem;
+    font-family: monospace;
+    font-style: normal;
+    color: #458588; /* accent-400 */
+    margin-bottom: 2px;
+  }
+
+  /* Hide the footnote fallback section on wide screens */
+  article.has-sidenotes .footnotes-sep,
+  article.has-sidenotes section.footnotes {
+    display: none;
+  }
+}
diff --git a/eleventy.config.js b/eleventy.config.js
index 84f904e..86f79a9 100644
--- a/eleventy.config.js
+++ b/eleventy.config.js
@@ -9,6 +9,7 @@ import markdownItAnchor from "markdown-it-anchor";
 import markdownItFootnote from "markdown-it-footnote";
 import syntaxHighlight from "@11ty/eleventy-plugin-syntaxhighlight";
 import { minify } from "html-minifier-terser";
+import posthtml from "posthtml";
 import { minify as minifyJS } from "terser";
 import registerUnfurlShortcode, { getCachedCard, prefetchUrl } from "./lib/unfurl-shortcode.js";
 import matter from "gray-matter";
@@ -489,6 +490,110 @@ export default function (eleventyConfig) {
     return result;
   });
 
+  // Sidenotes — convert markdown-it-footnote output into margin sidenotes.
+  // Wide screens (xl+): sidenotes float left. Narrow: footnote section at bottom.
+  eleventyConfig.addTransform("sidenotes", async function (content, outputPath) {
+    // Fast bail-outs
+    if (typeof outputPath !== "string" || !outputPath.endsWith(".html")) return content;
+    if (!content.includes('class="footnote-ref"')) return content;
+    const isPostPage = /\/(articles|notes|bookmarks|photos|replies|reposts|likes|pages)\/[^/]+\/index\.html$/.test(outputPath);
+    if (!isPostPage) return content;
+
+    const result = await posthtml([
+      (tree) => {
+        // 1. Build map: fnId → inline HTML (backref stripped, <p> wrappers stripped)
+        const fnMap = {};
+        tree.walk(node => {
+          if (
+            node.tag === "li" &&
+            node.attrs?.class?.includes("footnote-item") &&
+            node.attrs?.id
+          ) {
+            const fnId = node.attrs.id;
+            // Collect children, skip <a class="footnote-backref">
+            const children = (node.content || []).flatMap(child => {
+              if (child.tag === "p") {
+                // Strip outer <p>, keep inner content (excluding backref <a>)
+                return (child.content || []).filter(c =>
+                  !(c.tag === "a" && c.attrs?.class?.includes("footnote-backref"))
+                );
+              }
+              return [child];
+            });
+            fnMap[fnId] = children;
+          }
+          return node;
+        });
+
+        // 2. Track whether any sidenotes were injected
+        let hasSidenotes = false;
+
+        // 3. Replace each <sup class="footnote-ref"> with sidenote-host + aside
+        tree.walk(node => {
+          if (node.tag === "sup" && node.attrs?.class?.includes("footnote-ref")) {
+            // Find the child <a> to get href and id
+            const anchor = (node.content || []).find(c => c.tag === "a");
+            if (!anchor) return node;
+
+            const href = anchor.attrs?.href || ""; // e.g. "#fn1"
+            const fnId = href.replace(/^#/, "");   // e.g. "fn1"
+            const refId = anchor.attrs?.id || "";   // e.g. "fnref1"
+
+            // Extract numeric label from anchor text e.g. "[1]" → "1" or "[1:1]" → "1"
+            const rawLabel = (anchor.content || []).find(c => typeof c === "string") || "";
+            const label = rawLabel.replace(/[\[\]]/g, "").replace(/:.*$/, "").trim();
+
+            const noteContent = fnMap[fnId] || [];
+            if (!noteContent.length) return node;  // Skip orphan refs with no definition
+            hasSidenotes = true;
+
+            return {
+              tag: "span",
+              attrs: { class: "sidenote-host" },
+              content: [
+                {
+                  tag: "span",
+                  attrs: { class: "footnote-ref-num", id: refId },
+                  content: [label],
+                },
+                {
+                  tag: "aside",
+                  attrs: {
+                    class: "sidenote",
+                    "aria-label": `Sidenote ${label}`,
+                  },
+                  content: [
+                    {
+                      tag: "span",
+                      attrs: { class: "sidenote-number" },
+                      content: [label],
+                    },
+                    " ",
+                    ...noteContent,
+                  ],
+                },
+              ],
+            };
+          }
+          return node;
+        });
+
+        // 4. Add has-sidenotes class to <article> if any sidenotes were injected
+        if (hasSidenotes) {
+          tree.walk(node => {
+            if (node.tag === "article") {
+              const existing = node.attrs?.class || "";
+              node.attrs = { ...node.attrs, class: (existing + " has-sidenotes").trim() };
+            }
+            return node;
+          });
+        }
+      },
+    ]).process(content, { sync: false });
+
+    return result.html;
+  });
+
   // HTML minification — only during initial build, skip during watch rebuilds
   eleventyConfig.addTransform("htmlmin", async function (content, outputPath) {
     if (outputPath && outputPath.endsWith(".html") && process.env.ELEVENTY_RUN_MODE === "build") {
diff --git a/package-lock.json b/package-lock.json
index 73e31c7..fed4176 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -44,6 +44,7 @@
         "autoprefixer": "^10.4.0",
         "postcss": "^8.4.0",
         "postcss-cli": "^11.0.0",
+        "posthtml": "^0.16.7",
         "tailwindcss": "^3.4.0"
       }
     },
diff --git a/package.json b/package.json
index 991e831..5d2903b 100644
--- a/package.json
+++ b/package.json
@@ -47,6 +47,7 @@
     "autoprefixer": "^10.4.0",
     "postcss": "^8.4.0",
     "postcss-cli": "^11.0.0",
+    "posthtml": "^0.16.7",
     "tailwindcss": "^3.4.0"
   }
 }