diff --git a/docs/404.md b/docs/404.md
new file mode 100644
index 000000000..20948d579
--- /dev/null
+++ b/docs/404.md
@@ -0,0 +1,73 @@
+<script type="module">
+
+if (location.pathname.endsWith("/")) {
+  location.replace(`${location.pathname.slice(0, -1)}.html${location.search}${location.hash}`);
+}
+
+</script>
+
+# Page not found
+
+Sorry, but we can’t find the page you requested.
+
+Here’s something inspired by [Bridget Riley](https://en.wikipedia.org/wiki/Bridget_Riley) instead.
+
+```js
+import * as d3 from "npm:d3";
+
+const height = Math.min(640, width);
+const point = (cx, cy, r, a) => [cx + r * Math.cos(a), cy + r * Math.sin(a)];
+const circles = [];
+const random = d3.randomLcg(42);
+const n = 80;
+let a = 0.2;
+let x = width / 2;
+let y = height / 2;
+let r = Math.hypot(width, height) / 2;
+let dr = r / 6.5;
+
+while (r > 0) {
+  circles.push({x, y, r, a});
+  const t = random() * 2 * Math.PI;
+  const s = Math.sqrt((random() * dr * dr) / 4);
+  x += Math.cos(t) * s;
+  y += Math.sin(t) * s;
+  r -= dr;
+  a = -a;
+}
+
+const canvas = display(document.createElement("canvas"));
+canvas.width = width * devicePixelRatio;
+canvas.height = height * devicePixelRatio;
+canvas.style.width = `${width}px`;
+
+const context = canvas.getContext("2d");
+context.scale(devicePixelRatio, devicePixelRatio);
+
+(function frame(elapsed) {
+  context.save();
+  context.clearRect(0, 0, width, height);
+  context.translate(width / 2, height / 2);
+  context.rotate(Math.sin(elapsed / 50000));
+  context.translate(-width / 2, -height / 2);
+  context.beginPath();
+  for (let i = 0; i < n; ++i) {
+    let move = true;
+    d3.pairs(circles, ({x: x1, y: y1, r: r1, a: a1}, {x: x2, y: y2, r: r2, a: a2}) => {
+      const ai = ((i * 2) / n) * Math.PI;
+      context[move ? ((move = false), "moveTo") : "lineTo"](...point(x1, y1, r1, a1 + ai));
+      context.lineTo(...point(x2, y2, r2, a2 + ai));
+    });
+    d3.pairs(circles.slice().reverse(), ({x: x1, y: y1, r: r1, a: a1}, {x: x2, y: y2, r: r2, a: a2}) => {
+      const ai = ((i * 2 + 1) / n) * Math.PI;
+      context.lineTo(...point(x1, y1, r1, a1 + ai));
+      context.lineTo(...point(x2, y2, r2, a2 + ai));
+    });
+    context.closePath();
+  }
+  context.fillStyle = getComputedStyle(canvas).getPropertyValue("color");
+  context.fill();
+  context.restore();
+  if (canvas.isConnected) requestAnimationFrame(frame);
+})();
+```
diff --git a/src/navigation.ts b/src/navigation.ts
index 39fd270a2..5ade6c3f0 100644
--- a/src/navigation.ts
+++ b/src/navigation.ts
@@ -11,7 +11,7 @@ export async function readPages(root: string): Promise<NonNullable<RenderOptions
   if (config?.pages) return config?.pages;
   const pages: RenderOptions["pages"] = [];
   for await (const file of visitFiles(root)) {
-    if (extname(file) !== ".md") continue;
+    if (file === "404.md" || extname(file) !== ".md") continue;
     let parsed: ParseResult;
     try {
       parsed = parseMarkdown(await readFile(join(root, file), "utf-8"), root, file);
diff --git a/src/preview.ts b/src/preview.ts
index 9a5e4f73c..d509d7299 100644
--- a/src/preview.ts
+++ b/src/preview.ts
@@ -47,6 +47,7 @@ class Server {
 
   _handleRequest: RequestListener = async (req, res) => {
     console.log(req.method, req.url);
+    let pages;
     try {
       const url = new URL(req.url!, "http://localhost");
       let {pathname} = url;
@@ -124,7 +125,7 @@ class Server {
         // Otherwise, serve the corresponding Markdown file, if it exists.
         // Anything else should 404; static files should be matched above.
         try {
-          const pages = await readPages(this.root); // TODO cache? watcher?
+          pages = await readPages(this.root); // TODO cache? watcher?
           const {html} = await renderPreview(await readFile(path + ".md", "utf-8"), {
             root: this.root,
             path: pathname,
@@ -141,6 +142,20 @@ class Server {
     } catch (error) {
       console.error(error);
       res.statusCode = isHttpError(error) ? error.statusCode : 500;
+      if (req.method === "GET" && res.statusCode === 404) {
+        try {
+          const {html} = await renderPreview(await readFile(join(this.root, "404.md"), "utf-8"), {
+            root: this.root,
+            path: "/404",
+            pages,
+            resolver: this._resolver!
+          });
+          end(req, res, html, "text/html");
+          return;
+        } catch {
+          // ignore secondary error (e.g., no 404.md); show the original 404
+        }
+      }
       res.setHeader("Content-Type", "text/plain; charset=utf-8");
       res.end(error instanceof Error ? error.message : "Oops, an error occurred");
     }
diff --git a/src/render.ts b/src/render.ts
index eb6f26bb4..ad09fa625 100644
--- a/src/render.ts
+++ b/src/render.ts
@@ -55,7 +55,7 @@ function render(
   {path, pages, title, preview, hash, resolver}: RenderOptions & RenderInternalOptions
 ): string {
   return `<!DOCTYPE html>
-<meta charset="utf-8">
+<meta charset="utf-8">${path === "/404" ? `\n<base href="/">` : ""}
 <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
 ${
   parseResult.title || title