diff --git a/chrome/content.js b/chrome/content.js
index 9b0232e..3517ab1 100644
--- a/chrome/content.js
+++ b/chrome/content.js
@@ -201,9 +201,132 @@
       sendResponse({ items: harvestLinks(request.containerSelector) });
     } else if (request.action === 'scrapeData') {
       sendResponse({ items: scrapeData(request.selector, request.attribute) });
+    } else if (request.action === 'exportConversationMarkdown') {
+      exportConversationMarkdown().then(sendResponse).catch((error) => {
+        sendResponse({ error: error.message || 'Could not export the conversation.' });
+      });
+      return true;
     }
   });
 
+  async function exportConversationMarkdown() {
+    // ponytail: read the message nodes already loaded by ChatGPT; add virtual-scroll
+    // pagination only if ChatGPT starts unloading older messages from the DOM.
+    const messageElements = [...document.querySelectorAll('[data-message-author-role]')]
+      .filter((el) => ['user', 'assistant', 'system'].includes(el.dataset.messageAuthorRole));
+    if (!messageElements.length) throw new Error('No ChatGPT messages were found on this page.');
+
+    const imagesByElement = new Map();
+    const imagesByUrl = new Map();
+    let imageNumber = 0;
+
+    for (const message of messageElements) {
+      for (const image of message.querySelectorAll('img')) {
+        const url = image.currentSrc || image.src;
+        if (!url) continue;
+        let imageFile = imagesByUrl.get(url);
+        if (!imageFile) {
+          imageFile = await readConversationImage(url, ++imageNumber);
+          imagesByUrl.set(url, imageFile);
+        }
+        imagesByElement.set(image, imageFile);
+      }
+    }
+
+    const title = (document.querySelector('main h1')?.textContent || document.title || 'ChatGPT Conversation')
+      .replace(/\s*\|\s*ChatGPT.*$/i, '').trim() || 'ChatGPT Conversation';
+    const markdown = [`# ${title.replace(/^#+\s*/, '').replace(/\n/g, ' ')}`, ''];
+
+    messageElements.forEach((message) => {
+      const role = message.dataset.messageAuthorRole === 'assistant' ? 'ChatGPT' :
+        message.dataset.messageAuthorRole[0].toUpperCase() + message.dataset.messageAuthorRole.slice(1);
+      const body = cleanConversationMarkdown(nodeToConversationMarkdown(message, imagesByElement));
+      if (body) markdown.push(`## ${role}`, '', body, '');
+    });
+
+    return { markdown: `${markdown.join('\n').trim()}\n`, images: [...imagesByUrl.values()] };
+  }
+
+  async function readConversationImage(url, number) {
+    const fallbackName = `image-${String(number).padStart(3, '0')}.${imageExtension(url)}`;
+    try {
+      let bytes;
+      let mimeType = '';
+      if (url.startsWith('data:')) {
+        const match = url.match(/^data:([^,]*),(.*)$/s);
+        if (!match) throw new Error('Invalid data URL');
+        mimeType = (match[1].split(';')[0] || '').toLowerCase();
+        bytes = match[1].includes(';base64')
+          ? base64ToBytes(match[2])
+          : new TextEncoder().encode(decodeURIComponent(match[2]));
+      } else {
+        const response = await fetch(url, { credentials: 'include' });
+        if (!response.ok) throw new Error(`HTTP ${response.status}`);
+        mimeType = response.headers.get('content-type') || '';
+        bytes = new Uint8Array(await response.arrayBuffer());
+      }
+      const name = `image-${String(number).padStart(3, '0')}.${imageExtension(url, mimeType)}`;
+      return { name, url, mimeType, data: bytesToBase64(bytes) };
+    } catch (error) {
+      return { name: fallbackName, url, error: error.message || 'download failed' };
+    }
+  }
+
+  function nodeToConversationMarkdown(node, imagesByElement) {
+    if (node.nodeType === Node.TEXT_NODE) return node.nodeValue.replace(/\s+/g, ' ');
+    if (node.nodeType !== Node.ELEMENT_NODE) return '';
+
+    const tag = node.tagName.toLowerCase();
+    if (['button', 'svg', 'script', 'style', 'textarea'].includes(tag)) return '';
+    if (tag === 'img') {
+      const image = imagesByElement.get(node);
+      return image ? `![${node.alt || 'Image'}](images/${image.name})` : '';
+    }
+    if (tag === 'pre') return `\n\n\`\`\`\n${node.textContent.trim()}\n\`\`\`\n\n`;
+    if (tag === 'br') return '\n';
+    if (tag === 'a') {
+      const text = cleanConversationMarkdown([...node.childNodes]
+        .map((child) => nodeToConversationMarkdown(child, imagesByElement)).join(''));
+      return text ? `[${text}](${node.href})` : node.href;
+    }
+    if (tag === 'code') return `\`${node.textContent.trim()}\``;
+
+    const children = [...node.childNodes]
+      .map((child) => nodeToConversationMarkdown(child, imagesByElement)).join('');
+    if (/^h[1-6]$/.test(tag)) return `\n\n${'#'.repeat(Number(tag[1]))} ${children.trim()}\n\n`;
+    if (tag === 'li') return `\n- ${children.trim()}\n`;
+    if (['p', 'div', 'section', 'article', 'blockquote', 'ul', 'ol'].includes(tag)) {
+      return `\n${children}\n`;
+    }
+    return children;
+  }
+
+  function cleanConversationMarkdown(text) {
+    return text.replace(/[ \t]+\n/g, '\n').replace(/\n{3,}/g, '\n\n').trim();
+  }
+
+  function imageExtension(url, mimeType = '') {
+    const mimeExtensions = { 'image/jpeg': 'jpg', 'image/png': 'png', 'image/gif': 'gif',
+      'image/webp': 'webp', 'image/svg+xml': 'svg', 'image/bmp': 'bmp', 'image/avif': 'avif' };
+    if (mimeExtensions[mimeType.split(';')[0].toLowerCase()]) return mimeExtensions[mimeType.split(';')[0].toLowerCase()];
+    const match = url.match(/\.([a-z0-9]+)(?:[?#]|$)/i);
+    return match && ['jpg', 'jpeg', 'png', 'gif', 'webp', 'svg', 'bmp', 'avif'].includes(match[1].toLowerCase())
+      ? match[1].toLowerCase() : 'bin';
+  }
+
+  function base64ToBytes(value) {
+    const binary = atob(value.replace(/\s/g, ''));
+    return Uint8Array.from(binary, (char) => char.charCodeAt(0));
+  }
+
+  function bytesToBase64(bytes) {
+    let binary = '';
+    for (let offset = 0; offset < bytes.length; offset += 0x8000) {
+      binary += String.fromCharCode(...bytes.subarray(offset, offset + 0x8000));
+    }
+    return btoa(binary);
+  }
+
   function scanMedia(mediaType, customExtensions) {
     const extMap = {
       images: ['jpg', 'jpeg', 'png', 'gif', 'webp', 'svg', 'bmp', 'avif'],
diff --git a/chrome/popup.html b/chrome/popup.html
index 2792369..afe8e5b 100644
--- a/chrome/popup.html
+++ b/chrome/popup.html
@@ -55,6 +55,12 @@
       </div>
 
       <div class="tab-pane fade" id="toolsPane" role="tabpanel">
+        <div class="tool-card mb-3" id="conversationMarkdownCard" style="display: none;">
+          <h6>💬 ChatGPT Conversation</h6>
+          <button class="btn btn-sm btn-primary" id="downloadConversationMarkdownBtn">Download Conversation Markdown</button>
+          <div id="conversationMarkdownStatus" class="tool-results small text-muted mt-2"></div>
+        </div>
+
         <div class="tool-card mb-3">
           <h6>📥 Bulk Media Downloader</h6>
           <div class="d-flex gap-2 mb-2">
diff --git a/chrome/popup.js b/chrome/popup.js
index 19adf8d..ce1d450 100644
--- a/chrome/popup.js
+++ b/chrome/popup.js
@@ -609,11 +609,114 @@ function importRules() {
 // ---------------------------------------------------------------------
 
 function setupToolsTab() {
+  setupConversationMarkdown();
   setupMediaDownloader();
   setupLinkHarvester();
   setupDataScraper();
 }
 
+async function setupConversationMarkdown() {
+  const card = document.getElementById('conversationMarkdownCard');
+  const button = document.getElementById('downloadConversationMarkdownBtn');
+  const status = document.getElementById('conversationMarkdownStatus');
+  const tab = await getActiveTab();
+  if (!tab || !/^https:\/\/(chatgpt\.com|chat\.openai\.com)(?:\/|$)/i.test(tab.url || '')) return;
+
+  card.style.display = 'block';
+  button.addEventListener('click', async () => {
+    button.disabled = true;
+    status.textContent = 'Reading conversation…';
+    try {
+      const result = await chrome.tabs.sendMessage(tab.id, { action: 'exportConversationMarkdown' });
+      if (!result || result.error) throw new Error(result && result.error || 'Could not read this conversation.');
+      if (!result.images || result.images.length === 0) {
+        downloadBlob(result.markdown, 'chatgpt-conversation.md', 'text/markdown');
+        status.textContent = 'Markdown downloaded.';
+        return;
+      }
+
+      status.textContent = `Preparing ${result.images.length} image(s)…`;
+      const files = [{ name: 'conversation.md', bytes: new TextEncoder().encode(result.markdown) }];
+      for (const image of result.images) {
+        const bytes = image.data ? base64ToBytes(image.data) : await fetchConversationImage(image.url);
+        files.push({ name: `images/${image.name}`, bytes });
+      }
+      downloadBlob(createStoredZip(files), 'chatgpt-conversation.zip', 'application/zip');
+      status.textContent = 'ZIP downloaded.';
+    } catch (error) {
+      status.textContent = error.message || 'Could not download the conversation.';
+    } finally {
+      button.disabled = false;
+    }
+  });
+}
+
+async function fetchConversationImage(url) {
+  const response = await fetch(url, { credentials: 'include' });
+  if (!response.ok) throw new Error(`Could not download image (${response.status}).`);
+  return new Uint8Array(await response.arrayBuffer());
+}
+
+function base64ToBytes(value) {
+  const binary = atob(value);
+  return Uint8Array.from(binary, (char) => char.charCodeAt(0));
+}
+
+function createStoredZip(files) {
+  // ponytail: stored entries keep this dependency-free; add deflate only if archive size matters.
+  const parts = [];
+  const centralDirectory = [];
+  let offset = 0;
+
+  files.forEach(({ name, bytes }) => {
+    const nameBytes = new TextEncoder().encode(name);
+    const checksum = crc32(bytes);
+    const localHeader = new Uint8Array(30 + nameBytes.length);
+    const localView = new DataView(localHeader.buffer);
+    localView.setUint32(0, 0x04034b50, true);
+    localView.setUint16(4, 20, true);
+    localView.setUint32(14, checksum, true);
+    localView.setUint32(18, bytes.length, true);
+    localView.setUint32(22, bytes.length, true);
+    localView.setUint16(26, nameBytes.length, true);
+    localHeader.set(nameBytes, 30);
+    parts.push(localHeader, bytes);
+
+    const centralHeader = new Uint8Array(46 + nameBytes.length);
+    const centralView = new DataView(centralHeader.buffer);
+    centralView.setUint32(0, 0x02014b50, true);
+    centralView.setUint16(4, 20, true);
+    centralView.setUint16(6, 20, true);
+    centralView.setUint32(16, checksum, true);
+    centralView.setUint32(20, bytes.length, true);
+    centralView.setUint32(24, bytes.length, true);
+    centralView.setUint16(28, nameBytes.length, true);
+    centralView.setUint32(42, offset, true);
+    centralHeader.set(nameBytes, 46);
+    centralDirectory.push(centralHeader);
+    offset += localHeader.length + bytes.length;
+  });
+
+  const centralSize = centralDirectory.reduce((size, part) => size + part.length, 0);
+  const end = new Uint8Array(22);
+  const endView = new DataView(end.buffer);
+  endView.setUint32(0, 0x06054b50, true);
+  endView.setUint16(8, files.length, true);
+  endView.setUint16(10, files.length, true);
+  endView.setUint32(12, centralSize, true);
+  endView.setUint32(16, offset, true);
+  return new Blob([...parts, ...centralDirectory, end], { type: 'application/zip' });
+}
+
+function crc32(bytes) {
+  let crc = 0xffffffff;
+  for (const byte of bytes) {
+    crc ^= byte;
+    for (let bit = 0; bit < 8; bit++) crc = (crc >>> 1) ^ (crc & 1 ? 0xedb88320 : 0);
+  }
+  return (crc ^ 0xffffffff) >>> 0;
+}
+
 function setupMediaDownloader() {
   const mediaType = document.getElementById('mediaType');
   const mediaExtensions = document.getElementById('mediaExtensions');
diff --git a/firefox/content.js b/firefox/content.js
index 8480064..c546ec2 100644
--- a/firefox/content.js
+++ b/firefox/content.js
@@ -223,9 +223,132 @@
       sendResponse({ items: harvestLinks(request.containerSelector) });
     } else if (request.action === 'scrapeData') {
       sendResponse({ items: scrapeData(request.selector, request.attribute) });
+    } else if (request.action === 'exportConversationMarkdown') {
+      exportConversationMarkdown().then(sendResponse).catch((error) => {
+        sendResponse({ error: error.message || 'Could not export the conversation.' });
+      });
+      return true;
     }
   });
 
+  async function exportConversationMarkdown() {
+    // ponytail: read the message nodes already loaded by ChatGPT; add virtual-scroll
+    // pagination only if ChatGPT starts unloading older messages from the DOM.
+    const messageElements = [...document.querySelectorAll('[data-message-author-role]')]
+      .filter((el) => ['user', 'assistant', 'system'].includes(el.dataset.messageAuthorRole));
+    if (!messageElements.length) throw new Error('No ChatGPT messages were found on this page.');
+
+    const imagesByElement = new Map();
+    const imagesByUrl = new Map();
+    let imageNumber = 0;
+
+    for (const message of messageElements) {
+      for (const image of message.querySelectorAll('img')) {
+        const url = image.currentSrc || image.src;
+        if (!url) continue;
+        let imageFile = imagesByUrl.get(url);
+        if (!imageFile) {
+          imageFile = await readConversationImage(url, ++imageNumber);
+          imagesByUrl.set(url, imageFile);
+        }
+        imagesByElement.set(image, imageFile);
+      }
+    }
+
+    const title = (document.querySelector('main h1')?.textContent || document.title || 'ChatGPT Conversation')
+      .replace(/\s*\|\s*ChatGPT.*$/i, '').trim() || 'ChatGPT Conversation';
+    const markdown = [`# ${title.replace(/^#+\s*/, '').replace(/\n/g, ' ')}`, ''];
+
+    messageElements.forEach((message) => {
+      const role = message.dataset.messageAuthorRole === 'assistant' ? 'ChatGPT' :
+        message.dataset.messageAuthorRole[0].toUpperCase() + message.dataset.messageAuthorRole.slice(1);
+      const body = cleanConversationMarkdown(nodeToConversationMarkdown(message, imagesByElement));
+      if (body) markdown.push(`## ${role}`, '', body, '');
+    });
+
+    return { markdown: `${markdown.join('\n').trim()}\n`, images: [...imagesByUrl.values()] };
+  }
+
+  async function readConversationImage(url, number) {
+    const fallbackName = `image-${String(number).padStart(3, '0')}.${imageExtension(url)}`;
+    try {
+      let bytes;
+      let mimeType = '';
+      if (url.startsWith('data:')) {
+        const match = url.match(/^data:([^,]*),(.*)$/s);
+        if (!match) throw new Error('Invalid data URL');
+        mimeType = (match[1].split(';')[0] || '').toLowerCase();
+        bytes = match[1].includes(';base64')
+          ? base64ToBytes(match[2])
+          : new TextEncoder().encode(decodeURIComponent(match[2]));
+      } else {
+        const response = await fetch(url, { credentials: 'include' });
+        if (!response.ok) throw new Error(`HTTP ${response.status}`);
+        mimeType = response.headers.get('content-type') || '';
+        bytes = new Uint8Array(await response.arrayBuffer());
+      }
+      const name = `image-${String(number).padStart(3, '0')}.${imageExtension(url, mimeType)}`;
+      return { name, url, mimeType, data: bytesToBase64(bytes) };
+    } catch (error) {
+      return { name: fallbackName, url, error: error.message || 'download failed' };
+    }
+  }
+
+  function nodeToConversationMarkdown(node, imagesByElement) {
+    if (node.nodeType === Node.TEXT_NODE) return node.nodeValue.replace(/\s+/g, ' ');
+    if (node.nodeType !== Node.ELEMENT_NODE) return '';
+
+    const tag = node.tagName.toLowerCase();
+    if (['button', 'svg', 'script', 'style', 'textarea'].includes(tag)) return '';
+    if (tag === 'img') {
+      const image = imagesByElement.get(node);
+      return image ? `![${node.alt || 'Image'}](images/${image.name})` : '';
+    }
+    if (tag === 'pre') return `\n\n\`\`\`\n${node.textContent.trim()}\n\`\`\`\n\n`;
+    if (tag === 'br') return '\n';
+    if (tag === 'a') {
+      const text = cleanConversationMarkdown([...node.childNodes]
+        .map((child) => nodeToConversationMarkdown(child, imagesByElement)).join(''));
+      return text ? `[${text}](${node.href})` : node.href;
+    }
+    if (tag === 'code') return `\`${node.textContent.trim()}\``;
+
+    const children = [...node.childNodes]
+      .map((child) => nodeToConversationMarkdown(child, imagesByElement)).join('');
+    if (/^h[1-6]$/.test(tag)) return `\n\n${'#'.repeat(Number(tag[1]))} ${children.trim()}\n\n`;
+    if (tag === 'li') return `\n- ${children.trim()}\n`;
+    if (['p', 'div', 'section', 'article', 'blockquote', 'ul', 'ol'].includes(tag)) {
+      return `\n${children}\n`;
+    }
+    return children;
+  }
+
+  function cleanConversationMarkdown(text) {
+    return text.replace(/[ \t]+\n/g, '\n').replace(/\n{3,}/g, '\n\n').trim();
+  }
+
+  function imageExtension(url, mimeType = '') {
+    const mimeExtensions = { 'image/jpeg': 'jpg', 'image/png': 'png', 'image/gif': 'gif',
+      'image/webp': 'webp', 'image/svg+xml': 'svg', 'image/bmp': 'bmp', 'image/avif': 'avif' };
+    if (mimeExtensions[mimeType.split(';')[0].toLowerCase()]) return mimeExtensions[mimeType.split(';')[0].toLowerCase()];
+    const match = url.match(/\.([a-z0-9]+)(?:[?#]|$)/i);
+    return match && ['jpg', 'jpeg', 'png', 'gif', 'webp', 'svg', 'bmp', 'avif'].includes(match[1].toLowerCase())
+      ? match[1].toLowerCase() : 'bin';
+  }
+
+  function base64ToBytes(value) {
+    const binary = atob(value.replace(/\s/g, ''));
+    return Uint8Array.from(binary, (char) => char.charCodeAt(0));
+  }
+
+  function bytesToBase64(bytes) {
+    let binary = '';
+    for (let offset = 0; offset < bytes.length; offset += 0x8000) {
+      binary += String.fromCharCode(...bytes.subarray(offset, offset + 0x8000));
+    }
+    return btoa(binary);
+  }
+
   function scanMedia(mediaType, customExtensions) {
     const extMap = {
       images: ['jpg', 'jpeg', 'png', 'gif', 'webp', 'svg', 'bmp', 'avif'],
diff --git a/firefox/popup.html b/firefox/popup.html
index 2792369..afe8e5b 100644
--- a/firefox/popup.html
+++ b/firefox/popup.html
@@ -55,6 +55,12 @@
       </div>
 
       <div class="tab-pane fade" id="toolsPane" role="tabpanel">
+        <div class="tool-card mb-3" id="conversationMarkdownCard" style="display: none;">
+          <h6>💬 ChatGPT Conversation</h6>
+          <button class="btn btn-sm btn-primary" id="downloadConversationMarkdownBtn">Download Conversation Markdown</button>
+          <div id="conversationMarkdownStatus" class="tool-results small text-muted mt-2"></div>
+        </div>
+
         <div class="tool-card mb-3">
           <h6>📥 Bulk Media Downloader</h6>
           <div class="d-flex gap-2 mb-2">
diff --git a/firefox/popup.js b/firefox/popup.js
index d99ee5f..be63ed0 100644
--- a/firefox/popup.js
+++ b/firefox/popup.js
@@ -609,11 +609,114 @@ function importRules() {
 // ---------------------------------------------------------------------
 
 function setupToolsTab() {
+  setupConversationMarkdown();
   setupMediaDownloader();
   setupLinkHarvester();
   setupDataScraper();
 }
 
+async function setupConversationMarkdown() {
+  const card = document.getElementById('conversationMarkdownCard');
+  const button = document.getElementById('downloadConversationMarkdownBtn');
+  const status = document.getElementById('conversationMarkdownStatus');
+  const tab = await getActiveTab();
+  if (!tab || !/^https:\/\/(chatgpt\.com|chat\.openai\.com)(?:\/|$)/i.test(tab.url || '')) return;
+
+  card.style.display = 'block';
+  button.addEventListener('click', async () => {
+    button.disabled = true;
+    status.textContent = 'Reading conversation…';
+    try {
+      const result = await browser.tabs.sendMessage(tab.id, { action: 'exportConversationMarkdown' });
+      if (!result || result.error) throw new Error(result && result.error || 'Could not read this conversation.');
+      if (!result.images || result.images.length === 0) {
+        downloadBlob(result.markdown, 'chatgpt-conversation.md', 'text/markdown');
+        status.textContent = 'Markdown downloaded.';
+        return;
+      }
+
+      status.textContent = `Preparing ${result.images.length} image(s)…`;
+      const files = [{ name: 'conversation.md', bytes: new TextEncoder().encode(result.markdown) }];
+      for (const image of result.images) {
+        const bytes = image.data ? base64ToBytes(image.data) : await fetchConversationImage(image.url);
+        files.push({ name: `images/${image.name}`, bytes });
+      }
+      downloadBlob(createStoredZip(files), 'chatgpt-conversation.zip', 'application/zip');
+      status.textContent = 'ZIP downloaded.';
+    } catch (error) {
+      status.textContent = error.message || 'Could not download the conversation.';
+    } finally {
+      button.disabled = false;
+    }
+  });
+}
+
+async function fetchConversationImage(url) {
+  const response = await fetch(url, { credentials: 'include' });
+  if (!response.ok) throw new Error(`Could not download image (${response.status}).`);
+  return new Uint8Array(await response.arrayBuffer());
+}
+
+function base64ToBytes(value) {
+  const binary = atob(value);
+  return Uint8Array.from(binary, (char) => char.charCodeAt(0));
+}
+
+function createStoredZip(files) {
+  // ponytail: stored entries keep this dependency-free; add deflate only if archive size matters.
+  const parts = [];
+  const centralDirectory = [];
+  let offset = 0;
+
+  files.forEach(({ name, bytes }) => {
+    const nameBytes = new TextEncoder().encode(name);
+    const checksum = crc32(bytes);
+    const localHeader = new Uint8Array(30 + nameBytes.length);
+    const localView = new DataView(localHeader.buffer);
+    localView.setUint32(0, 0x04034b50, true);
+    localView.setUint16(4, 20, true);
+    localView.setUint32(14, checksum, true);
+    localView.setUint32(18, bytes.length, true);
+    localView.setUint32(22, bytes.length, true);
+    localView.setUint16(26, nameBytes.length, true);
+    localHeader.set(nameBytes, 30);
+    parts.push(localHeader, bytes);
+
+    const centralHeader = new Uint8Array(46 + nameBytes.length);
+    const centralView = new DataView(centralHeader.buffer);
+    centralView.setUint32(0, 0x02014b50, true);
+    centralView.setUint16(4, 20, true);
+    centralView.setUint16(6, 20, true);
+    centralView.setUint32(16, checksum, true);
+    centralView.setUint32(20, bytes.length, true);
+    centralView.setUint32(24, bytes.length, true);
+    centralView.setUint16(28, nameBytes.length, true);
+    centralView.setUint32(42, offset, true);
+    centralHeader.set(nameBytes, 46);
+    centralDirectory.push(centralHeader);
+    offset += localHeader.length + bytes.length;
+  });
+
+  const centralSize = centralDirectory.reduce((size, part) => size + part.length, 0);
+  const end = new Uint8Array(22);
+  const endView = new DataView(end.buffer);
+  endView.setUint32(0, 0x06054b50, true);
+  endView.setUint16(8, files.length, true);
+  endView.setUint16(10, files.length, true);
+  endView.setUint32(12, centralSize, true);
+  endView.setUint32(16, offset, true);
+  return new Blob([...parts, ...centralDirectory, end], { type: 'application/zip' });
+}
+
+function crc32(bytes) {
+  let crc = 0xffffffff;
+  for (const byte of bytes) {
+    crc ^= byte;
+    for (let bit = 0; bit < 8; bit++) crc = (crc >>> 1) ^ (crc & 1 ? 0xedb88320 : 0);
+  }
+  return (crc ^ 0xffffffff) >>> 0;
+}
+
 function setupMediaDownloader() {
   const mediaType = document.getElementById('mediaType');
   const mediaExtensions = document.getElementById('mediaExtensions');