Examine source code of (X)HTML5 Checker

{
  "name": "(X)HTML5 Checker",
  "description": "A small extension to validate the markup of current webpage",
  "manifest_version": 2,
  "version": "0.0.4resigned1",
  "permissions": [
    "<all_urls>",
    "activeTab"
  ],
  "icons": {
    "16": "assets/icon_16.png",
    "128": "assets/icon_128.png"
  },
  "browser_action": {
    "default_title": "Validate this page's HTML",
    "default_popup": "popup.html",
    "default_icon": {
      "16": "assets/icon_16.png",
      "128": "assets/icon_128.png"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "@xhtml5checker"
    }
  }
}