To be able to generate file from sources you need to install nodejs, gulp and some addons.
1 - Install current nodejs LTS (v10.15.3)
2 - Install gulp-cli globaly npm install --global gulp-cli
3 - Install gulp globaly npm install --global gulp@4.0.0
4 - Install all addons npm install

You should have something like this : 

E:\github\ESP3D-WEBUI>node -v
v10.14.0

E:\github\ESP3D-WEBUI>gulp -v
[22:17:03] CLI version 2.0.1
[22:17:03] Local version 4.0.0

E:\github\ESP3D-WEBUI>npm -v
6.4.1
5 - Do the code modification you want, then launch gulp package. 
E:\github\ESP3D-WEBUI>gulp package
[15:17:13] Using gulpfile E:\github\ESP3D-WEBUI\gulpfile.js
[15:17:13] Starting 'package'...
[15:17:13] Starting 'clean'...
[15:17:13] Finished 'clean' after 5.6 ms
[15:17:13] Starting 'lint'...
[15:17:13] Finished 'lint' after 144 ms
[15:17:13] Starting 'Copy'...
[15:17:13] Finished 'Copy' after 21 ms
[15:17:13] Starting 'concatApp'...
[15:17:14] Finished 'concatApp' after 53 ms
[15:17:14] Starting 'includehtml'...
[15:17:14] Finished 'includehtml' after 7.91 ms
[15:17:14] Starting 'includehtml'...
[15:17:14] Finished 'includehtml' after 6.18 ms
[15:17:14] Starting 'replaceSVG'...
[15:17:14] Finished 'replaceSVG' after 5.36 ms
[15:17:14] Starting 'minifyApp'...
\style.css: 125691
\style.css: 102958
[15:17:16] Finished 'minifyApp' after 2.2 s
[15:17:16] Starting 'smoosh'...
[15:17:16] Finished 'smoosh' after 117 ms
[15:17:16] Starting 'compress'...
[15:17:16] Finished 'compress' after 26 ms
[15:17:16] Starting 'clean2'...
[15:17:16] Finished 'clean2' after 3.83 ms
[15:17:16] Finished 'package' after 2.61 s
If no issue, a file called index.html.gz will be generated at the root of repository, so you just need to upload it like described in installation page

By default is build a multilanguage file, if file is too big for your flash size you can build with only english and one additional language:

  • gulp package --lang en will only build english
  • gulp package --lang fr will only build french + english
  • gulp package --lang es will only build spanish + english
  • gulp package --lang it will only build italian + english
  • gulp package --lang de will only build german + english
  • etc...