Why is my Lottie bigger than the MP4 I made it from?
Because a video codec compresses across time — it stores what changed between frames — while a Lottie holding images stores each frame whole, then base64 adds roughly a third on top. A growth of several times over is normal and not a fault in the conversion. Lottie makes sense when a runtime requires it, not as a way to save weight.
Can I keep 25 fps and still get under 2 MB?
Often not. At full frame rate a ten second clip needs 250 separate images, and dividing 2 MB between them leaves so little per frame that the artefacts become obvious. Halving the frame rate doubles the budget for every remaining frame, which usually looks better than a full-rate file compressed to bits. The tool tests both against your actual footage rather than guessing.
What does "lossless" actually mean here?
When frames repeat exactly — a hold, a static section — the file stores one copy and points every repeat at it. Those frames come out bit-for-bit identical. That work happens first, and if it gets you under target on its own, the pixels are never re-compressed at all.
Why won't my MP4 load?
Almost always the codec. Browsers decode 8-bit 4:2:0 H.264; they don't handle HEVC reliably, 10-bit, 4:2:2, 4:4:4 or ProRes. The converter inspects the file header and names the exact problem, then gives you an ffmpeg command with your filename already in it.
Why does the video tab give me a WebM instead of an MP4?
A web page can only use the encoder the browser provides. Safari and recent Chrome can write MP4; other browsers only write WebM. The tool checks before it starts and tells you which you'll get, rather than renaming a WebM and hoping you don't notice.
Do my files get uploaded?
No. There is no server. Every file is read, decoded and re-encoded inside your browser tab. You can disconnect from the network after the page loads and everything still works.