1. Not Checking Browser Support

Never assume every browser supports every HTML5 API. APIs like File System Access, WebGPU, Web Bluetooth, and Battery Status have limited support.

// Good

if ("clipboard" in navigator) {
  navigator.clipboard.writeText("Hello");
}

2. Ignoring Feature Detection

Detect API availability instead of checking browser names. Feature detection creates more reliable and future-proof applications.

3. Forgetting HTTPS Requirements

Many APIs only work in secure contexts, including:

• Clipboard API
• Geolocation API
• Notifications API
• MediaDevices API
• Service Workers
• Web Bluetooth

Always deploy production applications over HTTPS.

4. Requesting Permissions Too Early

Avoid requesting camera, microphone, notifications, or location permissions immediately after page load. Wait until users initiate an action.

5. Ignoring User Privacy

Only collect location, media, clipboard, or device information when necessary. Explain why permission is needed before requesting it.

6. Not Handling Permission Denials

Users may deny permission requests. Your application should continue to function gracefully instead of crashing.

try {
  const stream =
    await navigator.mediaDevices.getUserMedia({
      video: true
    });
} catch (error) {
  console.log("Permission denied.");
}

7. Blocking the Main Thread

Heavy image processing, parsing, or calculations should run inside Web Workers instead of freezing the user interface.

8. Using Local Storage for Sensitive Data

Never store passwords, authentication tokens, credit card information, or other sensitive data in Local Storage because JavaScript can access it.

9. Forgetting Error Handling

APIs such as Fetch, Clipboard, Geolocation, IndexedDB, and File APIs can fail. Always use try...catch or Promise error handling.

try {
  const response =
    await fetch("/api/users");
} catch (error) {
  console.error(error);
}

10. Ignoring Accessibility

Canvas, custom media players, drag-and-drop interfaces, and custom controls should include keyboard support, ARIA attributes, and screen reader alternatives.

11. Forgetting Cleanup

Remove event listeners, close Broadcast Channels, terminate Web Workers, and stop Media Streams when they are no longer needed to prevent memory leaks.

12. Not Optimizing Performance

Loading large files, images, videos, or datasets without optimization can reduce application performance.

• Lazy load media
• Compress images
• Use caching
• Virtualize large lists
• Use requestAnimationFrame() for animations

13. Relying on Deprecated APIs

Avoid deprecated browser APIs such as:

Deprecated API	Modern Alternative
document.execCommand()	Clipboard API
Application Cache	Service Workers + Cache API
Sync XHR	Fetch API

14. Skipping Progressive Enhancement

Core functionality should work even when advanced APIs are unavailable. Enhance the experience only when browser support exists.

15. Not Testing Across Browsers

Always verify HTML5 APIs in Chrome, Edge, Firefox, Safari, Android, and iOS browsers because implementation details can vary.