This is the source of the Constellation project website which is generated by jekyll and hosted by GitHub Pages.
To view locally, install the dependencies:
PATH="/usr/local/opt/ruby/bin:$PATH" bundle install
And invoke jekyll to serve it:
PATH="/usr/local/opt/ruby/bin:$PATH" bundle exec jekyll serve
Adjust PATH as appropriate – this happens to work on my macOS system.
To check validity:
PATH="/usr/local/opt/ruby/bin:$PATH" bundle exec jekyll build
PATH="/usr/local/opt/ruby/bin:$PATH" bundle exec htmlproofer --assume-extension ./_site