
I initially read this as a chapter title, which seemed like a bug. But it looks like what actually happened is just that the system that creates those prev/next links doesn't understand what to do when it's at the bottom of the very first chapter.
Not sure if this is worth calling out yet, since the book is still very much a WIP, but may be worth changing the label for that very first page from "Programming Design Systems" to something like "Title page" or removing the Previous Chapter link altogether for Chapter 1.