Examine source code of display toc on page

{
  "browser_specific_settings": {
    "gecko": {
      "id": "table-contents-extention@example.com"
    }
  },
  "manifest_version": 3,
  "name": "table-contents-extention",
  "version": "0.1",
  "description": "これは目次を表示するブラウザ拡張機能です。",
  "icons": {
    "32": "icons/icon-32.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "styles.css"
      ]
    }
  ],
  "optional_permissions": [
    "activeTab"
  ]
}