I hope the centrality of the internet makes this book timely. But the web is a dynamic, ever-changing place. The first web browser came into being in 1989 and it did not support colors, images, styling, or scripting. Three decades of evolution driven by market forces, implementation quirks, and the ever-expanding reach of the web made what browsers what they are today; those forces still matter. I both hope that this book will remain valuable for years to come, and also hope for dramatic change that makes it obsolete.
This book ignores modern developments like HTTP 2+ and Web Assembly; perhaps a later edition of this book will have to cover them. Nor does this book talk much about hardware virtualization like Web Bluetooth or format centralization like web video. Maybe in the future those will be more central to what browsers do.
Many dedicated, brilliant, and talented engineers have devoted themselves to the web over its first three decades. The structure of the web embeds their ideas, inventions, and taste. Even if, in a few years, this book is obsolete or out-dated, I hope it still passes along those lessons.