Examine source code of A simple online piano

{
  "manifest_version": 2,
  "name": "Piano Virtual Keyboard",
  "version": "1.0.0",
  "description": "Play a virtual piano keyboard directly from your browser.",
  "permissions": [
    "activeTab",
    "storage"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_title": "Piano Virtual Keyboard",
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content_script.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "applications": {
    "gecko": {
      "id": "piano-virtual-keyboard@example.com"
    }
  }
}