Phone doesn't vibrate if screen is locked when using HTML5 Vibration API

I have a website (app, but still in browser only), where I've implemented some vibrations on particular events using HTML5 Vibration API.

All works fine until screen is locked. Then the vibration stops and if it's continuous - continue when I unlock the screen again. Any ideas?

I've tested on Android 4.2.2, and Chrome Mobile 42 (version 42.0.2311.108).