Beyond Their Time opens HEIC photos in the browser with the following free software, used unmodified.

heic-to 1.5.2 (the "csp" build)
  Source:  https://github.com/hoppergee/heic-to
  Licence: GNU Lesser General Public License, version 3 or later — LICENSE.txt

libheif 1.22.2 (compiled into heic-to)
  Source:  https://github.com/strukturag/libheif/tree/v1.22.2
  Licence: GNU Lesser General Public License, version 3 — libheif-COPYING.txt (the project's own COPYING file)

libde265 1.0.16 (compiled into libheif)
  Source:  https://github.com/strukturag/libde265/tree/v1.0.16
  Licence: GNU Lesser General Public License, version 3 — libde265-COPYING.txt (the project's own COPYING file)

The GNU LGPL version 3 incorporates the terms of the GNU General Public License, version 3: GPL-3.0.txt.