The super cool website you just saw is an electronic version of a physical book that I made and used to propose to my significant other. He thought it was pretty great! It's based on the lyrics of a song by The Magnetic Fields.
The physical book contains real flowers and heart-shaped boxes (okay, they're more like heart-shaped envelopes) in Chapter 7. Most of the hearts have cute photographs inside! The electronic version of the book just contains scans of these pages, minus the photos. On the other hand, the physical book contains guitar tabs, while the electronic version contains actual music.
The website uses HTML, CSS, and JavaScript to mimic the original book layout, which used the action of flipping the pages to create pacing. The website works best in Chrome.