Examine source code of Timeline clock

{
  "manifest_version": 3,
  "browser_specific_settings": {
    "gecko": {
      "id": "timelinewatch@jschwegler.com"
    }
  },
  "name": "Timeline clock",
  "version": "1.0.1",
  "action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "assets/icon16.png",
      "48": "assets/icon48.png",
      "128": "assets/icon128.png"
    }
  },
  "permissions": [
    "activeTab"
  ]
}