Examine source code of osu! preview

{
  "manifest_version": 2,
  "name": "osu! preview",
  "description": "Preview an osu! beatmap in your browser.",
  "version": "0.0.6",
  "icons": {
    "48": "icon48.png",
    "128": "icon128.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "*://osu.ppy.sh/*"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "page_action": {
    "default_icon": "icon48.png",
    "default_popup": "popup.html"
  },
  "permissions": [
    "tabs",
    "https://*.ppy.sh/",
    "http://*.ppy.sh/"
  ]
}