We Built a Video Rendering Engine by Lying to the Browser About What Time It Is

A video rendering engine was built to turn any web page into a deterministic video file, a task designed to support an AI agent. The core problem is that browsers are real-time systems, leading to stuttering when trying to record complex animations. To solve this, the system virtualizes time by patching key browser APIs like `requestAnimationFrame` and `setTimeout`, making the browser believe time only advances when instructed. This approach also required complex workarounds, such as intercepting `<video>` elements for server-side transcoding and spying on Web Audio API calls to reconstruct the audio track separately with FFmpeg.