01. basic marker placeholder

A single declarative <?marker> processing instruction is replaced when a matching <template for> arrives later in the same response. The connection stays open between the shell and the late fragment. No JavaScript runs on the page.

The marker, initially showing a skeleton

What was streamed in

<template for="bio">
  <h2>Paul Kinlan</h2>
  <p>Chrome DevRel...</p>
</template>

Hint: the server holds the connection open for ~1.8s then flushes the template. The shell renders immediately.

learn more

Chrome blog: Declarative Partial Updates — Barry Pollard's introduction with worked examples.
WICG explainer: patching — Why <?start>/<?end> and <template for> exist and how they compose.
template-for polyfill — Works in browsers that don't yet support it.
WICG/declarative-partial-updates — Specification repo, where issues and discussion live.

source

Everything the server runs for this example. Each file links to GitHub so you can copy or fork it.

handler.ts/app/src/examples/01-basic-marker/handler.ts

import { sleep, streamingResponse } from "../../lib/streaming.ts";
import { loadShell, readSibling, tryStatic } from "../../lib/files.ts";

const SHELL = loadShell(import.meta.url);
const LATE = readSibling(import.meta.url, "late.html");

export default function handle(_req: Request, path: string): Response | Promise<Response> {
  if (path === "/" || path === "/index.html") {
    return streamingResponse(async (write) => {
      await write(SHELL);
      await sleep(1800);
      await write(LATE);
    });
  }
  return tryStatic(import.meta.url, path) ?? new Response("Not found", { status: 404 });
}

open on GitHub →

index.html/app/src/examples/01-basic-marker/index.html

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>01 · Basic marker</title>
  <link rel="stylesheet" href="/public/styles.css">
</head>
<body>
<main>
  <p class="crumbs"><a href="/">&larr; back to index</a></p>
  <h1>01. basic marker placeholder</h1>
  <p class="lede">A single declarative <code>&lt;?marker&gt;</code> processing instruction is replaced when a matching <code>&lt;template for&gt;</code> arrives later in the same response. The connection stays open between the shell and the late fragment. No JavaScript runs on the page.</p>

  <h3>The marker, initially showing a skeleton</h3>
  <section class="demo-area" id="bio">
    <?start name="bio">
      <div class="skeleton lg"></div>
      <div class="skeleton"></div>
      <div class="skeleton sm"></div>
    <?end>
  </section>

  <h3>What was streamed in</h3>
  <pre><code>&lt;template for="bio"&gt;
  &lt;h2&gt;Paul Kinlan&lt;/h2&gt;
  &lt;p&gt;Chrome DevRel...&lt;/p&gt;
&lt;/template&gt;</code></pre>

  <p style="margin-top:2rem;color:var(--muted);font-size:.85rem;">Hint: the server holds the connection open for ~1.8s then flushes the template. The shell renders immediately.</p>

  <section class="refs">
    <h3>learn more</h3>
    <ul>
      <li><a href="https://developer.chrome.com/blog/declarative-partial-updates?hl=en" target="_blank" rel="noopener">Chrome blog: Declarative Partial Updates</a> <span class="note">— Barry Pollard's introduction with worked examples.</span></li>
      <li><a href="https://github.com/WICG/declarative-partial-updates/blob/main/patching-explainer.md" target="_blank" rel="noopener">WICG explainer: patching</a> <span class="note">— Why <code>&lt;?start&gt;/&lt;?end&gt;</code> and <code>&lt;template for&gt;</code> exist and how they compose.</span></li>
      <li><a href="https://github.com/GoogleChromeLabs/template-for-polyfill" target="_blank" rel="noopener">template-for polyfill</a> <span class="note">— Works in browsers that don't yet support it.</span></li>
      <li><a href="https://github.com/WICG/declarative-partial-updates" target="_blank" rel="noopener">WICG/declarative-partial-updates</a> <span class="note">— Specification repo, where issues and discussion live.</span></li>
    </ul>
  </section>

  <!-- source-viewer goes here -->

  <footer class="byline">made by <a href="https://paul.kinlan.me/" target="_blank" rel="noopener">Paul Kinlan</a></footer>
</main>

open on GitHub →

late.html/app/src/examples/01-basic-marker/late.html

<template for="bio">
  <h2 style="margin:0 0 .4rem;">Paul Kinlan</h2>
  <p style="margin:0;color:var(--muted);">Chrome DevRel. Likes the web, agents, and pubs in Liverpool.</p>
</template>
</body>
</html>

open on GitHub →

lib/streaming.ts/app/src/lib/streaming.ts

export class StreamAborted extends Error {
  constructor() {
    super("stream aborted by client");
    this.name = "StreamAborted";
  }
}

export type ChunkSource = (write: (chunk: string) => Promise<void>) => Promise<void>;

const encoder = new TextEncoder();

export function streamingResponse(
  source: ChunkSource,
  contentType = "text/html; charset=utf-8",
): Response {
  let aborted = false;
  const body = new ReadableStream<Uint8Array>({
    async start(controller) {
      const write = async (chunk: string) => {
        if (aborted) throw new StreamAborted();
        try {
          controller.enqueue(encoder.encode(chunk));
        } catch {
          aborted = true;
          throw new StreamAborted();
        }
      };
      try {
        await source(write);
      } catch (err) {
        if (!(err instanceof StreamAborted)) console.error("stream source error", err);
      } finally {
        if (!aborted) {
          try {
            controller.close();
          } catch {
            // already closed
          }
        }
      }
    },
    cancel() {
      aborted = true;
    },
  });
  return new Response(body, {
    headers: {
      "content-type": contentType,
      "cache-control": "no-store",
      "x-content-type-options": "nosniff",
    },
  });
}

export function sleep(ms: number): Promise<void> {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

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lib/files.ts/app/src/lib/files.ts

// File helpers for examples: read sibling files, render the page shell with the
// source viewer injected, and serve sibling static assets (CSS/JS/HTML) on demand.

import { sourceBlock } from "./source.ts";

const MIME: Record<string, string> = {
  css: "text/css; charset=utf-8",
  js: "application/javascript; charset=utf-8",
  html: "text/html; charset=utf-8",
  svg: "image/svg+xml",
  json: "application/json; charset=utf-8",
};

function dirFor(handlerUrl: string): string {
  return new URL(".", handlerUrl).pathname;
}

export function readSibling(handlerUrl: string, name: string): string {
  return Deno.readTextFileSync(dirFor(handlerUrl) + name);
}

// Reads a sibling HTML file. The marker `<!-- source-viewer goes here -->` is replaced
// with the source-viewer block. Extra named placeholders ({{name}}) can be supplied via `vars`.
const SOURCE_MARKER = "<!-- source-viewer goes here -->";

export function loadShell(
  handlerUrl: string,
  name = "index.html",
  vars: Record<string, string> = {},
): string {
  let raw = readSibling(handlerUrl, name);
  raw = raw.replace(SOURCE_MARKER, sourceBlock(handlerUrl));
  for (const [k, v] of Object.entries(vars)) {
    raw = raw.replaceAll(`{{${k}}}`, v);
  }
  return raw;
}

// Try to serve a sibling static file from the example folder. Returns null when the
// file is missing, the path escapes the folder, or the path is empty.
export function tryStatic(handlerUrl: string, requestPath: string): Response | null {
  const safe = requestPath.replace(/^\/+/, "");
  if (!safe || safe.includes("..")) return null;
  // Never let the browser fetch handler.ts or README.md through this fallback.
  if (safe === "handler.ts" || safe === "README.md") return null;
  try {
    const content = Deno.readFileSync(dirFor(handlerUrl) + safe);
    const ext = safe.split(".").pop() ?? "";
    return new Response(content, {
      headers: {
        "content-type": MIME[ext] ?? "application/octet-stream",
        "cache-control": "no-store",
      },
    });
  } catch {
    return null;
  }
}

open on GitHub →