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High Performance Web Sites by Steve Souders

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In eighth grade, my history class studied the efficiency experts of the Industrial Revolution. I was enthralled by the techniques they used to identify and overcome bottlenecks in manufacturing. The most elegant improvement, in my mind, was the adjustable stepstool that afforded workers of different heights the ability to more easily reach the conveyor belt—a simple investment that resulted in improved performance for the life of the process.

Three decades later, I enjoy comparing the best practices in this book to that 19th-century stepstool. These best practices enhance an existing process. They require some upfront investment, but the cost is small—especially in comparison to the gains. And once these improvements are put in place, they continue to boost performance over the life of the development process. I hope you’ll find these rules for building high performance web sites to be elegant improvements that benefit you and your users.

How This Book Is Organized

After two quick introductory chapters, I jump into the main part of this book: the 14 performance rules. Each rule is described, one per chapter, in priority order. Not every rule applies to every site, and not every site should apply a rule the same way, but each is worth considering. The final chapter of this book shows how to analyze web pages from a performance perspective, including some case studies.

Chapter 1, explains that at least 80 percent of the time it takes to display a web page happens after the HTML document has been downloaded, and describes the importance of the techniques in this book.

Chapter 2, provides a short description of HTTP, highlighting the parts that are relevant to performance.

Chapter 3, describes why extra HTTP requests have the biggest impact on performance, and discusses ways to reduce these HTTP requests including image maps, CSS sprites, inline images using data: URLs, and combining scripts and stylesheets.

Chapter 4, highlights the advantages of using a content delivery network.

Chapter 5, digs into how a simple HTTP header dramatically improves your web pages by using the browser’s cache.

Chapter 6, explains how compression works and how to enable it for your web servers, and discusses some of the compatibility issues that exist today.

Chapter 7, reveals how stylesheets affect the rendering of your page.

Chapter 8, shows how scripts affect rendering and downloading in the browser.

Chapter 9, discusses the use of CSS expressions and the importance of quantifying their impact.

Chapter 10, talks about the tradeoffs of inlining your JavaScript and CSS versus putting them in external files.

Chapter 11, highlights the often-overlooked impact of resolving domain names.

Chapter 12, quantifies the benefits of removing whitespace from your JavaScript.

Chapter 13, warns against using redirects, and provides alternatives that you can use instead.

Chapter 14, reveals what happens if a script is included twice in a page.

Chapter 15, describes how ETags work and why the default implementation is bad for anyone with more than one web server.

Chapter 16, emphasizes the importance of keeping these performance rules in mind when using Ajax.

Chapter 17, gives examples of how to identify performance improvements in real-world web sites.

Conventions Used in This Book

The following typographical conventions are used in this book:


Indicates new terms, URLs, email addresses, filenames, file extensions, pathnames, directories, Unix utilities, and general emphasis.

Constant width

Indicates computer code in a broad sense. This includes commands, options, switches, variables, attributes, keys, functions, types, classes, namespaces, methods, modules, properties, parameters, values, objects, events, event handlers, XML tags, HTML tags, macros, the contents of files, and the output from commands.

HTTP requests and responses are designated graphically as shown in the following example.

GET / HTTP/1.1 is an HTTP request header
HTTP/1.1 200 OK is an HTTP response header

Code Examples

Online examples can be found on this book’s companion web site:


Examples are included in each chapter in the context in which they are discussed. They are also listed here for easy review.

No Image Map (Chapter 3)


Image Map (Chapter 3)


CSS Sprites (Chapter 3)


Inline Images (Chapter 3)


Inline CSS Images (Chapter 3)


Separate Scripts (Chapter 3)


Combined Scripts (Chapter 3)


CDN (Chapter 4)


No CDN (Chapter 4)


No Expires (Chapter 5)


Far Future Expires (Chapter 5)


Nothing Gzipped (Chapter 6)


HTML Gzipped (Chapter 6)


Everything Gzipped (Chapter 6)


CSS at the Bottom (Chapter 7)


CSS at the Top (Chapter 7)


CSS at the Top Using @import (Chapter 7)


CSS Flash of Unstyled Content (Chapter 7)


Scripts in the Middle (Chapter 8)


Scripts Block Downloads (Chapter 8)


Scripts at the Top (Chapter 8)


Scripts at the Bottom (Chapter 8)


Scripts Top vs. Bottom (Chapter 8)


Deferred Scripts (Chapter 8)


Expression Counter (Chapter 9)


One-Time Expressions (Chapter 9)


Event Handler (Chapter 9)


Inlined JS and CSS (Chapter 10)


External JS and CSS (Chapter 10)


Cacheable External JS and CSS (Chapter 10)


Post-Onload Download (Chapter 10)


Dynamic Inlining (Chapter 10)


Small Script Normal (Chapter 12)


Small Script Minified (Chapter 12)


Small Script Obfuscated (Chapter 12)


Large Script Normal (Chapter 12)


Large Script Minified (Chapter 12)


Large Script Obfuscated (Chapter 12)


XMLHttpRequest Beacon (Chapter 13)


Image Beacon (Chapter 13)


Duplicate Scripts—Not Cached (Chapter 14)


Duplicate Scripts—Cached (Chapter 14)


Duplicate Scripts—10 Cached (Chapter 14)


In general, you may use the code in this book and these online examples in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission.

We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: "High Performance Web Sites by Steve Souders. Copyright 2007 Steve Souders, 978-0-596-52930-7.”

If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at .

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Ash Patel and Geoff Ralston were the Yahoo! executives who asked me to start a center of expertise focused on performance. Several Yahoo!s helped answer questions and discuss ideas: Ryan Troll, Doug Crockford, Nate Koechley, Mark Nottingham, Cal Henderson, Don Vail, and Tenni Theurer. Andy Oram, my editor, struck the balance of patience and prodding necessary for a first-time author. Several people helped review the book: Doug Crockford, Havi Hoffman, Cal Henderson, Don Knuth, and especially Jeffrey Friedl, Alexander Kirk, and Eric Lawrence.

This book was completed predominantly in spare hours on the weekends and late at night. I thank my wife and daughters for giving me those hours on the weekends to work. I thank my parents for giving me the work ethic to do the late-night hours.

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