How to build a drift-free fullscreen countdown timer using vanilla JavaScript

A developer building a fullscreen countdown timer for blankscreen.io found that the standard setInterval approach causes time drift, especially in background tabs. To fix this, they used performance.now() with requestAnimationFrame to measure actual elapsed time instead of counting ticks. The Screen Wake Lock API was integrated to prevent devices from sleeping mid-countdown, with a visibility change listener to re-request the lock if the tab is backgrounded. Rather than loading an external audio file, a synthesized two-tone alarm was generated on demand using the Web Audio API, avoiding autoplay policy issues. The article shares reusable code snippets for each of these three techniques as progressive enhancements for any browser-based timer project.