Check the element before you blame the codec
A hidden video element can look exactly like a decoder problem when the layout is the actual failure.
A media bug once sent the investigation straight toward encoding. The file loaded. The browser reported no useful decode error. The frame stayed blank. The problem was not inside the video at all: the element had been laid out where nobody could see it.
Start with the thing the user can see.
Before changing formats, inspect the rendered box. Does it have width and height? Is it clipped, translated, covered, transparent, or sitting in a stacking context beneath another surface? Is the poster visible while the media is not? The geometry can answer those questions faster than a new transcode.
A blank rectangle encourages infrastructure thinking. But a visual failure can be produced by a perfectly healthy file inside an unhealthy layout.
Debug from the boundary inward.
I now check the chain in order: the request, the element, its intrinsic dimensions, its computed box, its visibility, the layers above it, and only then the decoder. Each step either proves the next investigation is necessary or removes it from the path.
This is the same habit I want in larger systems. Do not promote the most complicated explanation just because it sounds technical. Attach the claim to the nearest observable evidence.
The small lesson scales.
A product team can waste hours replacing the subsystem that was easiest to blame. Good debugging preserves the order of proof. Check the element. Check the boundary. Then change the machinery.