Play first, convert later: the first rule for strange formats
The reflex on receiving an MKV or AVI is "find a converter" — but transcoding costs time and always loses something. The smarter order is play first: see what is inside, then decide whether to touch it.
Why play first
Transcoding is lossy and time-consuming: every generation costs quality, and wrong parameters make things worse. The right handling path depends entirely on content — a complete, playable file may never need converting (self-archival or local playback); footage that is already poor will not be rescued by converting, so do not spend the time.
Browser player tools exist for exactly this: no transcoding, no upload, no intermediate decode — a direct local preview. MKV, AVI, and FLV "will not open" mostly because default browser decoders do not recognize them; a local decode path plays them fine.
What each player tool is for
MKV player: the freest container — HEVC, AV1, multi-track audio and subtitles all live inside it. Play first to see the stream structure, then decide to extract or convert. AVI player: legacy from old devices and software; confirm the footage is worth processing. FLV player: Flash-era streams and tutorials; confirm content, then convert to MP4 for archiving.
M3U8 player: the index file of streamed video — paste the address to verify the link works and what it contains. Playing before downloading an m3u8 is the standard move. Pair playback with the codec checker: playback confirms content, inspection confirms parameters — together they complete the decision basis.
Three roads after preview
Self-view and archive: plays fine and quality is good — no conversion needed; formats only matter when you need to exchange files. Long-term compatibility: convert to H.264 MP4 once, done. Further processing: for editing or publishing, convert and compress to the target platform spec.
The core variable is "who watches": for yourself, compatibility is irrelevant; for others, H.264 MP4 is the only safe common language. Play-first makes that decision rest on the facts of content and quality, not the name of the format.