Examine source code of HTTP.cat as error pages

{
  "name": "HTTP.cat as error pages",
  "description": "Replaces all error pages with HTTP cat :3",
  "version": "1.0.0",
  "manifest_version": 2,
  "icons": {
    "64": "http.cat-64px.png",
    "128": "http.cat-128px.png",
    "256": "http.cat-256px.png",
    "512": "http.cat-512px.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "<all_urls>"
  ],
  "web_accessible_resources": [
    "index.html"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{ba007a9e-edac-4c77-a783-eb42c3d5dbf6}"
    }
  }
}