Examine source code of Readability Score

{
  "manifest_version": 2,
  "name": "Readability Score",
  "version": "2.1",
  "description": "Measure how hard a page is to read. Get a live Flesch Reading Ease score, a US grade level, words per sentence, word count and estimated reading time for the whole article or just your selection.",
  "icons": {
    "32": "icons/icon-32.png",
    "48": "icons/icon-48.png",
    "64": "icons/icon-64.png",
    "96": "icons/icon-96.png",
    "128": "icons/icon-128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{a6d5efa9-afc0-4212-a554-9707ee75182f}",
      "strict_min_version": "101.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_title": "Readability Score",
    "default_popup": "src/popup.html",
    "default_icon": {
      "32": "icons/icon-32.png",
      "48": "icons/icon-48.png",
      "64": "icons/icon-64.png"
    }
  }
}