The MIT Libraries WordPress multisite was a massive project I worked on for 1.5 years. Not only did I build out several new sites, I completely rebuilt and refactored the underlying theme in use. When I started on the site/theme, only some of the pages were fully responsive, but by the time the project ended, every page of the hundreds performed well on mobile devices. In addition to refactoring the PHP and JavaScript, I also spent a lot of the time reworking the SASS and it is the project I am most proud of.