In 2016 I was added to the small team working to maintain Energy.Gov's Drupal 7 installation, with the intention of leading the frontend work in implementing a "light brand refresh". This design refresh ended up being a complete, top-to-bottom redesign, wholly different from their previous theme. My team and I took my knowledge of Twig in Drupal 7 and built the new design from scratch, while adding an entirely new editing interface for Energy.Gov editors to create visually interesting and intricate pages. Our plan was to make content, styles, and even templates that could be easily migrated to Drupal 8. Most of that fell through, and I ended up leading a complete rebuild (again) in Drupal 8 a few years later. That project eventually launched in 2021, and my design can still be seen on energy.gov (as of the writing of this page).
With the huge amount of content on the site and the many, many moving parts of the design, we implemented visual regression testing via Backstop.js for approximately 80 key pages, and ran this with every deployment to check that only the changes we expected occurred. It was an excellent bellwether and I've since used it on several projects.
This was also my first foray into Drupal Paragraphs, and though we later concluded that Layout Builder would have been a much more appropriate approach for reasons ranging from user experience to server performance, I am very happy with the capabilities of the site and the sheer options editors have before them, which all results in a site that looks and feels very fluid and clean.