Examine source code of WatermarkPreview

{
  "manifest_version": 2,
  "name": "WatermarkPreview",
  "version": "1.0",
  "description": "Temporarily overlay text or logo watermarks on images in-page for previewing position and opacity before exporting.",
  "icons": {},
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "watermark.js"
      ],
      "css": [
        "watermark.css"
      ],
      "run_at": "document_end"
    }
  ],
  "permissions": [
    "storage",
    "activeTab"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{2a2c508c-16a6-49dd-9e27-89b5066c89bf}"
    }
  }
}