If you're a fan of Bootstrap like me, you know that Bootstrap is actually developed using LESS, and is then compiled down to CSS. If you want to make changes to Bootstrap, it's easiest to tweak the pre-compiled LESS files so your changes are propagated correctly into the resulting CSS. Harp is quite intelligent, and will automatically render LESS files into CSS on the fly. This means that while running Harp as a web server, your HTML pages can link in Bootstrap's LESS files as CSS without having to manually compile the LESS files; Harp handles it. Check out the video tutorial (especially around 6:53) to see how Harp works with Bootstrap. Again, it's absolutely brilliant, and a big timesaver when you want to customize the LESS variables for Bootstrap for your particular site.