十三月格式转换 is a modified deployment of VERT.
VERT source and contributors: https://github.com/VERT-sh/VERT
Site source: https://github.com/bigzhihui/VERT
VERT is licensed under AGPL-3.0; see /LICENSE and the site source.

Conversion software includes ImageMagick/magick-wasm, Pandoc/pandoc-wasm,
FFmpeg/ffmpeg.wasm, Mediabunny and its codec packages, and browser_wasi_shim.
Original copyright and license notices are retained in the repository,
bundled libraries and the corresponding upstream projects.

The self-hosted @ffmpeg/core 0.12.10 is licensed GPL-2.0-or-later.
Wrapper source: https://github.com/ffmpegwasm/ffmpeg.wasm/tree/main/packages/core
Core build source: https://github.com/ffmpegwasm/ffmpeg.wasm/tree/main/ffmpeg
FFmpeg source: https://github.com/FFmpeg/FFmpeg
FFmpeg WASM core licensing: https://ffmpegwasm.netlify.app/docs/overview/

GPL-2.0 text: /licenses/FFmpeg-GPL-2.0.txt