Examine source code of HTML Valid

{
  "manifest_version": 2,
  "name": "HTML Valid",
  "version": "0.1",
  "description": "HTML Valid is an extension that validates HTML pages in Firefox, providing instant feedback on errors through an easy-to-read icon.",
  "icons": {
    "48": "icon.png"
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "48": "icon.png"
    }
  },
  "permissions": [
    "activeTab"
  ],
  "background": {
    "scripts": [
      "popup.js"
    ],
    "persistent": false
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{8f91b21d-b2f4-4ff7-bc3c-b1866f04e8b3}"
    }
  }
}