Examine source code of Twitter Visible Alt Text

{
  "name": "Twitter Visible Alt Text",
  "version": "1.0.0",
  "description": "Makes alt text visible for sighted users",
  "manifest_version": 2,
  "icons": {
    "48": "icon-48.png",
    "96": "icon-96.png"
  },
  "content_scripts": [
    {
      "run_at": "document_start",
      "matches": [
        "https://twitter.com/*"
      ],
      "css": [
        "content.css"
      ],
      "js": [
        "content.js"
      ]
    }
  ]
}