A computer science degree traditionally includes courses in operating systems, compilers, and databases that replace mystery with code. These courses transform Linux, Postgres, and LLVM into improvements, additions, and optimizations of an understandable core architecture. The lesson transcends the specific system studied: all computer systems, no matter how big and seemingly complex, can be studied and understood.
But web browsers are still opaque, not just to students but to industry programmers and even to researchers. This book dissipates that mystery by systematically explaining all major components of a modern web browser.
Parts 1–3 of this book construct a basic browser weighing in at around 1000 lines of code, twice that after exercises. The average chapter takes 4–6 hours to read, implement, and debug for someone with a few years’ programming experience. Part 4 of this book covers advanced topics; those chapters are longer and have more code. The final browser weighs in at about 3000 lines.
Your browserThis book assumes that you will be building a web browser along the way while reading it. However, it does present nearly all the code—inlined into the book—for a working browser for every chapter. So most of the time, the book uses the term “our browser”, which refers to the conceptual browser we (you and us, the authors) have built so far. In cases where the book is referring specifically to the implementation you have built, the book says “your browser”. will “work” at each step of the way, and every chapter will build upon the last.This idea is from J. R. Wilcox, inspired in turn by S. Zdancewic’s course on compilers. That way, you will also practice growing and improving complex software. If you feel particularly interested in some component, please do flesh it out, complete the exercises, and add missing features. We’ve tried to arrange it so that this doesn’t make later chapters more difficult.
The code in this book uses Python 3, and
we recommend you follow along in the same. When the book shows Python
command lines, it calls the Python binary python3.This is for clarity. On some
operating systems, python means Python 3, but on others
that means Python 2. Check which version you have! That
said, the text avoids dependencies where possible and you can try to
follow along in another language. Make sure your language has libraries
for TLS connections (Python has one built in), graphics (the text uses
Tk, Skia, and SDL), and JavaScript evaluation (the text uses DukPy).
This book’s browser is irreverent toward standards: it handles only a sliver of the full HTML, CSS, and JavaScript languages, mishandles errors, and isn’t resilient to malicious inputs. It is also quite slow. Despite that, its architecture matches that of real browsers, providing insight into those 10 million line of code behemoths.
That said, we’ve tried to explicitly note when the book’s browser simplifies or diverges from standards. If you’re not sure how your browser should behave in some edge case, fire up your favorite web browser and try it out.
We’d like to recognize the countless people who built the web and the various web browsers. They are wonders of the modern world. Thank you! We learned a lot from the books and articles listed in this book’s bibliography—thank you to their authors. And we’re especially grateful to the many contributors to articles on Wikipedia (especially those on historic software, formats, and protocols). We are grateful for this amazing resource, one which in turn was made possible by the very thing this book is about.
Pavel: James R. Wilcox and I dreamed up this book during a late-night chat at ICFP 2018. Max Willsey proofread and helped sequence the chapters. Zach Tatlock encouraged me to develop the book into a course. And the students of CS 6968, CS 4962, and CS 4560 at the University of Utah found countless errors and suggested important simplifications. I am thankful to all of them. Most of all, I am thankful to my wife Sara, who supported my writing and gave me the strength to finish this many-year-long project.
Chris: I am eternally grateful to my wife Sara for patiently listening to my endless musings about the web, and encouraging me to turn my idea for a browser book into reality. I am also grateful to Dan Gildea for providing feedback on my browser-book concept on multiple occasions. Finally, I’m grateful to Pavel for doing the hard work of getting this project off the ground and allowing me to join the adventure. (Turns out Pavel and I had the same idea!)
This book is, and will remain, a work in progress. Please leave
comments and mark typos; the book has built-in feedback tools, which you
can enable with Ctrl-E (or Cmd-E on a Mac).
The full source code is also available on GitHub, though
we prefer to receive comments through the built-in tools.