Examine source code of WhatsApp Masker

{
  "manifest_version": 2,
  "name": "WhatsApp Masker",
  "description": "Hides WhatsApp Web messages until hovered.",
  "version": "1.0",
  "browser_specific_settings": {
    "gecko": {
      "id": "whatsapp-masker@mehdi",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "content_scripts": [
    {
      "matches": [
        "*://web.whatsapp.com/*"
      ],
      "css": [
        "mask.css"
      ],
      "js": [
        "mask.js"
      ]
    }
  ]
}