The web flasher at /flash/ uses or adapts the following works.

esptool-js 0.7.0 (vendor/esptool-js/bundle.js)
  Copyright (c) 2023 Espressif Systems (Shanghai) Co. Ltd.
  Apache License 2.0; full text in vendor/esptool-js/LICENSE.
  Source: https://github.com/espressif/esptool-js
  The file is the package's prebuilt bundle.js, unmodified
  (sha256 00ebe6fb0b202976d3e2a3c6b9a6bd438c26a81d2af6739c95ac276a64c1a84e).
  The bundle includes pako 2.1.0 and atob-lite 2.0.0, both MIT; their notices follow.

pako 2.1.0
  (The MIT License)
  Copyright (C) 2014-2017 by Vitaly Puzrin and Andrei Tuputcyn
  Source: https://github.com/nodeca/pako

atob-lite 2.0.0
  MIT License. Source: https://github.com/hughsk/atob-lite

ESP-IDF nvs_flash and esp-idf-nvs-partition-gen
  Copyright (c) 2015-2025 (nvs_flash), 2018-2023 (the generator)
  Espressif Systems (Shanghai) Co. Ltd.
  Apache License 2.0 (text in vendor/esptool-js/LICENSE).
  Sources: https://github.com/espressif/esp-idf (components/nvs_flash),
  https://github.com/espressif/esp-idf-nvs-partition-gen
  nvs.js reads the NVS page format as nvs_flash defines it and places
  entries as nvs_partition_gen.py does; the entry placement is ported from
  that tool's Page.write_varlen_data and write_varlen_binary_data.

ESPConnect
  The flasher's flow (confirm, write with progress and cancel, session log), its
  manual download-mode help, offsets read from the board's partition table and
  the boot slot readout follow ESPConnect. No ESPConnect source is copied; the
  otadata reader in image.js is written from ESP-IDF's bootloader_support.
  Source: https://github.com/thelastoutpostworkshop/ESPConnect

MIT License text, for pako, atob-lite and ESPConnect:

  ESPConnect: Copyright (c) 2025 The Last Outpost Workshop

  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:

  The above copyright notice and this permission notice shall be included in all
  copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.