HEIC input: libheif-js 1.23.2, LGPL-3.0. Bundled file is unmodified.
Package and documentation: https://www.npmjs.com/package/libheif-js/v/1.23.2
Wrapper source and build instructions: https://github.com/catdad-experiments/libheif-js
Underlying decoder source: https://github.com/strukturag/libheif
HEVC decoder source: https://github.com/strukturag/libde265
Licence: libheif-LICENSE.txt. Replacing libheif-bundle.js replaces the local decoder.

All files are served locally. This implementation decodes the primary still image to 8-bit RGBA; motion, depth data and HDR are not retained. It does not encode HEIC.
Verification fixture: https://github.com/strukturag/libheif/blob/master/examples/example.heic (test artifacts only).