Let's say you want Chrome to automatically download pieces of pages, such as Facebook comments. How could you do it? I recently wanted to do this and couldn't find docs, so here's what I did.

1. Make a folder somewhere for a browser extension.

2. In that folder, make manifest.json with contents like:

   {
     "name": "Downloader",
     "description": "Downloads stuff",
     "version": "1.0",
     "manifest_version": 3,
     "permissions": [
       "activeTab",
       "downloads"
     ],
     "background": {
       "service_worker": "background.js"
     },
     "content_scripts": [{
       "matches": [
         "https://example.com/path/*",
       ],
       "js": ["content_script.js"]
     }]
   }
   

3. In content_script.js put:

   // This file extracts what you want from the page
   // and asks the background script to save it.
   
   // You write the code to extract what you want.
   const yourStringToSave =
     yourFunctionToExtractFromPage();
   
   chrome.runtime.sendMessage(
       /* extension id not needed */ undefined,
       [fileNameToUse,
        yourStringToSave]);
   

4. In background.js put:

   // This file receives messages from the content
   // script and puts them in your Dowloads folder.
   
   function makeDataUrl(body) {
     // We use a data: url because Chrome has
     // trouble with object URLs in Incognito.
     return "data:application/json;base64," +
       btoa(unescape(encodeURIComponent(body)));
   }
   
   chrome.runtime.onMessage.addListener(
     function(message) {
       const fname = message[0];
       const body = message[1];
   
       chrome.downloads.download({
         conflictAction: "overwrite",
         filename: fname,
         url: makeDataUrl(body),
       });
     });
   

5. Visit chrome://extensions

6. Enable Developer Mode

7. Click the "Load unpacked" and select the extension directory

8. If you need it in Incognito, click "Details" on the extension card and then enable "Allow in Incognito".

Example code: github.

(And, yes, this is how Facebook comments are back.)