web/webpack.js.org/reference/site/plugins/internal-plugins/#apiplugin/index.html

<!DOCTYPE html>

<html lang="en">
<head>
<meta charset="utf-8"/>
<meta content="width=device-width, initial-scale=1" name="viewport"/>
<title>Page Not Available - Offline Mode</title>
<style>
* { margin: 0; padding: 0; box-sizing: border-box; }
body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  display: flex; align-items: center; justify-content: center;
  min-height: 100vh; background: #f5f5f7; color: #1d1d1f;
}
.container { text-align: center; padding: 60px 40px; max-width: 560px; }
.code { font-size: 120px; font-weight: 700; color: #d2d2d7; line-height: 1; margin-bottom: 16px; }
h1 { font-size: 28px; font-weight: 600; margin-bottom: 12px; }
p { font-size: 17px; color: #6e6e73; line-height: 1.5; margin-bottom: 24px; }
.info { font-size: 14px; color: #86868b; background: #fff; border-radius: 12px;
        padding: 16px 20px; margin-bottom: 24px; text-align: left; }
.info strong { color: #1d1d1f; }
a.btn {
  display: inline-block; padding: 12px 24px; background: #0071e3; color: #fff;
  text-decoration: none; border-radius: 8px; font-size: 15px; font-weight: 500;
}
a.secondary { color: #06c; text-decoration: none; font-size: 15px; margin-left: 16px; }
</style>
</head>
<body>
<div class="container">
<div class="code">404</div>
<h1>Page Not Available</h1>
<p>This page could not be downloaded for offline viewing.</p>
<div class="info">
<strong>Why am I seeing this?</strong><br/>
    This website has been cloned for offline/local use. Some pages cannot be fully replicated in offline mode.
  </div>
<a class="btn" href="javascript:history.back()">Go Back</a>
<a class="secondary" href="/">Home</a>
</div>
<script data-offline="true">
// Offline mode: basic interactivity
document.addEventListener('DOMContentLoaded', function() {
  // Make theme toggle work
  var themeBtn = document.querySelector('[class*="theme"], [aria-label*="theme"], [aria-label*="Theme"]');
  if (themeBtn) {
    themeBtn.addEventListener('click', function() {
      var html = document.documentElement;
      var current = html.getAttribute('data-theme') || 'light';
      html.setAttribute('data-theme', current === 'light' ? 'dark' : 'light');
    });
  }
  // Make sidebar toggles work
  document.querySelectorAll('[class*="sidebar"] details').forEach(function(d) {
    d.setAttribute('open', '');
  });
});
</script>
</body>
</html>