HTTP 200 isn't 'working' for video streams
A video link can answer 'OK' and still show a black screen.
how it works
HEAD/status probes miss empty/black streams; sample frames and pixel-check before marking channels alive.
Technique: A pattern or hard-won lesson, written down so it isn't learned twice.
The code for this one lives in a private repo. This page explains the idea so you can build your own.