There are many sites that showcase their simplicity with free software:

  https://www.palletsprojects.com/ (python / lektor)
    - source: https://github.com/pallets/website

  http://flask.pocoo.org/ (python / flask)
    - source: https://github.com/pallets/flask-website/

  https://developers.google.com/web/fundamentals/ (appengine / jekyll)
    - source: https://github.com/google/

Have you tried writing a progressive web app using Polymer?