AVI is a 30-year-old container, and that is the problem
AVI was Microsoft's 1992 answer to digital video. It is still around, but the codecs that ended up living inside it — DivX, Xvid, MPEG-4 Part 2, MP3 audio — are basically retired. Modern browsers will not decode them. Quick Look will not preview them. iOS will refuse them outright. Even VLC, the universal player of last resort, sometimes audio-desyncs on older AVIs. The right move is almost always to re-encode the streams into H.264 + AAC and hand them off in an MP4 wrapper, which is exactly what this tool does.
What this tool does, exactly
We re-encode the video to H.264 at CRF 22 (visually lossless for most home-video and screen-capture AVIs), the audio to AAC at 160 kbps, and write the result as an MP4 with the +faststart flag so it begins playing immediately when streamed. Pixel format is forced to yuv420p because some old AVIs use exotic chroma layouts that hardware decoders refuse. Quality stays the same since we are going from one lossy codec to another at high quality — no detail is recovered, but none is meaningfully lost either.
No upload — what does that mean?
Most online AVI converters are server-side: your file gets uploaded, transcoded on their box, and returned via download link. That is fine for a clip from a public DVD rip, less fine for old family-camcorder footage you would rather not park on a stranger's server. This tool runs FFmpeg inside your browser via WebAssembly. Your file never leaves your device — you can pull the network plug after the page loads and the conversion still finishes.
When the desktop app is the better answer
The browser version of FFmpeg is single-threaded with about a 2 GB memory ceiling. AVI re-encoding is heavier than a remux, so a 90-minute home-video AVI will take a while in here. If you are converting a folder of old camcorder tapes, the Clipy desktop app uses the native FFmpeg build with multi-threading and no memory cap. Same conversion, typically 3–5× faster, and still no upload.