"HTML5" is shorthand for the modern web platform: not just tags, but a huge set of browser Web APIs reachable from JavaScript. Geolocation, local storage, drag-and-drop, notifications, the clipboard, background workers, real-time channels — all built in.
These are what let a web page rival a native app. Each is its own object on window or navigator; you feature-detect (if ('geolocation' in navigator)) before using one, since support and permissions vary.
<!doctype html>
<html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px">
<pre id="out"></pre>
<script>
const apis = ['geolocation','clipboard'].map(k => k + ': ' + (k in navigator));
apis.push('localStorage: ' + ('localStorage' in window));
apis.push('serviceWorker: ' + ('serviceWorker' in navigator));
document.getElementById('out').textContent = apis.join('\n');
</script>
</body></html>