What Wasn’t Covered

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The last sixteen chapters have, I hope, given you a solid understanding of all of the major components of a web browser, from the network requests it makes to way it stores your data and keeps it safe. With such a vast topic I had to leave a few things out. Here’s my list of the most important things not covered by this book, in no particular order.

JavaScript Execution

A large part of a modern web browser is a very-high-performance implementation of JavaScript. Today, every major browser not only runs JavaScript, but compiles it, in flight, to low-level machine code using runtime type analysis. Plus, techniques like hidden classes infer structure where JavaScript doesn’t provide any, lowering memory usage and garbage collection pressure. On top of all of that, modern browsers also execute WebAssembly, a hardware-independent bytecode format target for many other programming languages that may one day be co-equal to JavaScript on the web.

This book skips building the JavaScript engine, instead using DukPy. I made this choice because while JavaScript execution is central to a modern browser, it uses techniques fairly similar to the execution of other languages like Python, Lua, or Java. The best way to learn about the insides of a modern JavaScript engine is a book on programming language implementation.

Text & Graphics Rendering

Text rendering is much more complex than it may seem at the surface. Letters differ in widths and heights. Accents may need to be stacked atop characters. Characters may change shape by proximity to other characters, like for ligatures or for shaping (for cursive fonts). Sometimes languages are written right-to-left or top-to-bottom. Then there are typographic features, like kerning and variants. But the most complex of all is hinting, which is a little computer program embedded in a font that modifies it to better match the discrete pixel grid. Text rendering of course affects Skia, but it also affects layout, determining the size and position of content on the screen.

And more broadly, graphics in general is pretty complex! Our toy browser uses Skia, which is the real rasterization engine of Chromium and some other browsers. But we didn’t really talk at all about how Skia actually works, and it turns out to be pretty complex. It not only renders text but applies all sorts of blends and effects quickly and with high quality on basically all CPUs and GPUs. In a real browser this becomes even more complex, with fancy compositing systems, graphics process security sandboxing, and various platform-specific font and OS compositing integrations. And there is a whole lot of additional effort to implementation lower-level JavaScript-exposed APIs like Canvas, WebGL, and WebGPU.

I skipped this topic in the book because high-quality implementations are available in libraries like Skia (for graphics) and Harfbuzz (for text), as well as various system libraries, so are arguably not browser-specific. But there is a depth here best served by a book on these specific subjects.

Connection Security & Privacy

Web browsers now ship with a sophisticated suite of cryptographic protocols with bewildering names like AES-GCM, ChaCha20, and HMAC-SHA512. These protocols protect against malicious actors with the ability to read or write network packets. At the broadest level, connection security is established via the TLS protocol (which cameos in Chapter 1) and maintained by an ecosystem of cryptographers, certificate authorities, and open-source projects.

I chose to skip an in-depth discussion of TLS because this book’s irreverent attitude toward completeness and validation is incompatible with real security engineering. A minimal and incomplete version of TLS is a broken and insecure version of it, contrary to the intended goal and pedagogically counterproductive. The best way to learn about modern cryptography and network security is a book on that topic.

Privacy on the web is another important topic that I skipped. In some ways security and privacy are related (and certainly complement one other), but they are not the same. And privacy on the web is in flux, such as debates around third-party cookies, fingerprinting, and whether there should be APIs to help with advertising. I chose to skip this topic because many basic concepts remain unsettled: what the standards of privacy are and what role governments, browser developers, website authors, and users should play in them.

Network Caching and Media

Caching makes network requests faster by skipping most of them. What makes it more than a mere optimization, however, is the extent to which HTTP is designed to enable caching. Implementing a network cache deepens one’s understanding of HTTP significantly. That said, the networking portion of this book is long enough, and at no point in the book did the lack of a cache feel painful, so I decided to leave this topic out.

And since the majority of network bandwidth and battery life is today eaten up by video playback and video conferencing, there is a whole world of complexity in real-time video encoding, decoding and rendering. Real browsers have large teams devoted to these services and APIs, and many researchers across the world work on video compression. Video codecs are fascinating, but again not very browser-specific, so this book skips it entirely, and I suggest dedicated books on these subjects.

Fancier Layout Modes

The layout algorithm used in real browsers is much more sophisticated than that covered in the book, with features like floating layout, positioned elements, flexible boxes, grids, tables, and more. Implementing these layout modes is complex and requires care and sophistication—especially if you want speed and incremental performance. Important techniques here include multi-phase layoutWe do a little bit of multi-phase layout in the book, with words in a line having their x, width, and height computed in the first phase and then their y computed in a separate phase based on the baseline. But we don’t talk much about it as an example of multi-phase layout, and real browsers have much more complex sets of layout phases. and measure-layout phases, with tricky caching strategies necessary to produce good performance.

I chose to skip fancier layout in this book because even the simple layout algorithm described here is quite complex, and real-world layout algorithms involve a lot of accidental complexity caused by old standards and backwards compatibility, which I didn’t want to talk much about.

Browser UIs and Developer tools

A real browser has a much more complex and powerful “browser UI”—meaning the chrome around the web page, where you can enter URLs, see tabs, and so on—than our toy browser. In fact, a large fraction of a real browser team works just on this, and not on the “web platform” itself. The multi-process nature of a modern browser also makes it difficult to interact with synchronous OS APIs, as we saw with accessibility in Chapter 14. Plus, many browsers (desktop ones, at least) support powerful extension APIs that enable developers to extend the browser UI. To help with that, browser UIs are often implemented in HTML and rendered by the browser itself.

Also, it’d be almost impossible to build complex web apps without some kind of debugging aid, so all real browsres have built-in debuggers. Believe it or not, for quite a long time browser engineers just did a lot of printf debugging. This changed in a big way with the innovative Firebug browser extension for Firefox, and eventually today’s integrated developer tools. These developer tools have deep integration with the browser engine itself to implement features like observing the styles of elements in real time or pausing and stepping through JavaScript execution.

I skipped this topic because many challenges in browser UI are the same those of as any other UI: design, usability, complexity, and so on. That would make for a tedious book. Even the debugger, conceptually quite interesting, is only useful if a substantial amount of UI work is done to make it usable. Unfortunately, I’m not aware of any book on developer tools, but many books will cover basic user interface development.


Real browsers have evolved an incredibly impressive array of testing techniques to ensure they maintain and improve quality over time. In total, they have batteries of hundreds of thousands of unit, and integration tests. Recently, a lot of focus has been put on robust cross-browser tests that allow a single automated test to run on all browsers to verify that they all behave the same on the same input. And there are now yearly Interoperability“Interop”, for short. benchmarks that measure how well browsers are doing against this goal for key features. Behind the scenes of testing is a whole world of code and infrastructure to efficiently run these tests continuously and provide extensive frameworks to make testing easy.