Examine source code of Youtube Preview

{
  "manifest_version": 2,
  "name": "Youtube Preview",
  "description": "Preview youtube videos with video thumbnails or video itself without leaving that page.",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "applications": {
    "gecko": {
      "id": "youtube-preview_m.qayyum@live.com",
      "strict_min_version": "42.0"
    }
  },
  "version": "1.3",
  "icons": {
    "128": "images/icon.png"
  },
  "browser_action": {
    "default_icon": "images/icon.png"
  },
  "permissions": [
    "tabs",
    "storage"
  ],
  "content_scripts": [
    {
      "js": [
        "content.js"
      ],
      "css": [
        "style.css"
      ],
      "matches": [
        "*://www.youtube.com/*"
      ],
      "run_at": "document_end"
    }
  ],
  "web_accessible_resources": [
    "images/*.png",
    "style.css"
  ]
}