404 error page with navigation options

A default server 404 response — raw text or a framework-generated HTML stub — gives users no path forward. They cannot navigate home, search for what they wanted, or understand whether the URL was ever valid. ISO 25010 reliability.recoverability requires the system to guide users back to a functional state; a missing custom 404 page abandons them entirely. For SEO, a well-structured custom 404 with navigation also prevents Google from interpreting broken links as site-wide quality signals.

Create app/not-found.tsx in Next.js App Router (or the equivalent for your framework) with at minimum a home link and a back button.

// app/not-found.tsx
import Link from 'next/link'

export default function NotFound() {
  return (
    <main style={{ textAlign: 'center', padding: '4rem' }}>
      <h1>Page not found</h1>
      <p>This URL doesn\'t exist or has moved.</p>
      <Link href="/">Go to home</Link>
    </main>
  )
}

Add a search input if your app has site search — it converts the 404 into a recovery path rather than a dead end.