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Why won't my screen recording play?
The three most common reasons, in order of frequency: an invalid duration written by MediaRecorder (fix with WebM to MP4), a container the target app doesn't support like WebM in Safari or PowerPoint (fix with the video converter), or a codec the target app can't decode. Drop your file in above to see exactly which one applies.
Skip the diagnostics entirely
None of this matters if you record and share with Clipy: recordings stream to the server as you record, get transcoded into a broadly compatible MP4 automatically, and viewers get an instant link instead of a file with its own container quirks.
The Infinity/NaN duration bug, explained
Chrome's MediaRecorder writes WebM files without a valid duration in the header while recording, because it doesn't know the final length until you stop. Many players read that missing value as Infinity or NaN, which breaks the seek bar and can make some apps refuse the file entirely. The fix isn't re-encoding the video — it's re-muxing the container so the correct duration gets written into the header. The WebM to MP4 converter does exactly that as part of the conversion.
Why the file extension can lie
A video's extension is just a filename convention; the actual format is determined by the first few bytes of the file (a WebM starts with an EBML signature, an MP4 starts with an ftyp box near the top). Renaming a .webm to .mp4 doesn't convert it — it just makes some apps trust the wrong player logic and either refuse the file or play it incorrectly. This checker reads the real bytes, so it catches that mismatch even when the filename looks correct.
Faststart and why it matters
An MP4 file is internally split into a `moov` atom (the index: where every frame lives) and an `mdat` atom (the actual frame data). If `moov` comes after `mdat`, a browser or streaming player has to download the whole file before it can start playing, because it doesn't know where anything is yet. "Faststart" means `moov` was moved to the front, so playback (and scrubbing) can begin immediately. Screen recorders and some export tools sometimes skip this step.
