提升网页性能：最佳实践

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"Even Faster Web Sites - 性能最佳实践指南，专为Web开发者设计，由Steve Souders撰写。" 《Even Faster Web Sites》这本书深入探讨了Web性能优化的策略和最佳实践，作者Steve Souders是Web性能领域的知名专家。这本书主要针对Web开发者，旨在帮助他们提升网站加载速度和用户体验，从而提高网站的访问量和用户满意度。书中涵盖了多个关键知识点： 1. **减少HTTP请求**：每个HTTP请求都会增加页面加载时间，因此减少请求次数至关重要。这可能涉及合并CSS和JavaScript文件，使用CSS Sprites技术来整合图像，或者利用HTTP/2的多路复用特性。 2. **压缩内容**：通过GZIP压缩可以显著减小HTML、CSS和JavaScript文件的大小，从而加快传输速度。服务器和浏览器都支持GZIP压缩，开发人员应确保其启用。 3. **缓存利用**：正确设置HTTP缓存头可以帮助浏览器缓存静态资源，避免不必要的重下载。理解Last-Modified、ETag和Cache-Control等HTTP头的使用是优化缓存的关键。 4. **延迟加载**：非首屏内容可以通过延迟加载（Lazy Loading）技术在用户滚动到相应位置时才加载，减少初始页面渲染的时间。 5. **优化图片**：压缩图片大小，使用适当的文件格式（如JPEG、PNG或WebP），以及使用响应式图片技术，可以显著减少页面重量。 6. **减少DNS查找**：过多的DNS查找会延长页面加载时间。通过合并域名或使用预加载DNS查找（DNS Prefetching）可以改善这一点。 7. **优先级处理**：确保关键内容如HTML、CSS和脚本优先加载，以便尽早呈现页面内容，提供即时反馈。 8. **利用浏览器缓存**：合理设置浏览器缓存策略，可以提高用户的回访速度，例如使用Service Worker实现离线缓存。 9. **代码优化**：精简JavaScript和CSS，移除不必要的代码，避免阻塞渲染的脚本执行，使用异步加载和模块化方法。 10. **测量与监控**：使用工具如Google PageSpeed Insights、Lighthouse或WebPageTest定期检查并优化网站性能，持续监控以识别潜在的性能瓶颈。 Steve Souders的《Even Faster Web Sites》提供了实用的建议和技巧，帮助开发者了解和应用这些最佳实践，以实现更高效的Web开发。通过实施这些策略，开发者可以构建更快、更响应的网站，提高用户满意度和搜索引擎排名。

• Rule 5: Put Stylesheets at the Top

• Rule 6: Put Scripts at the Bottom

• Rule 7: Avoid CSS Expressions

• Rule 8: Make JavaScript and CSS External

• Rule 9: Reduce DNS Lookups

• Rule 10: Minify JavaScript

• Rule 11: Avoid Redirects

• Rule 12: Remove Duplicate Scripts

• Rule 13: Configure ETags

• Rule 14: Make Ajax Cacheable

I call them “rules” because there is little ambiguity about their adoption. Consider these

statistics for the top 10 U.S. web sites

for March 2007:

• Two sites used CSS sprites.

• 26% of resources had a future Expires header.

• Five sites compressed their HTML, JavaScript, and CSS.

• Four sites minified their JavaScript.

The same statistics for April 2009 show that these rules are gaining traction:

• Nine sites use CSS sprites.

• 93% of resources have a future Expires header.

• Ten sites compress their HTML, JavaScript, and CSS.

• Nine sites minify their JavaScript.

The rules from High Performance Web Sites still apply and are where most web com-

panies should start. Progress is being made, but there’s still more work to be done on

this initial set of rules.

But the Web isn’t standing still, waiting for us to catch up. Although the 14 rules from

High Performance Web Sites still apply, the growth in web page content and Web 2.0

applications introduces a new set of performance challenges. Even Faster Web Sites

provides the best practices needed by developers to make these next-generation web

sites faster.

The chapters in this book are organized into three areas: JavaScript performance

(Chapters 1–7), network performance (Chapters 8–12), and browser performance

(Chapters 13 and 14). A roundup of the best tools for analyzing performance comes in

the Appendix.

AOL, eBay, Facebook, Google Search, Live Search, MSN.com, MySpace, Wikipedia, Yahoo!, and YouTube,

according to Alexa.

xiv | Preface

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Six of the chapters were written by contributing authors:

• Chapter 1, Understanding Ajax Performance, by Douglas Crockford

• Chapter 2, Creating Responsive Web Applications, by Ben Galbraith and Dion

Almaer

• Chapter 7, Writing Efficient JavaScript, by Nicholas C. Zakas

• Chapter 8, Scaling with Comet, by Dylan Schiemann

• Chapter 9, Going Beyond Gzipping, by Tony Gentilcore

• Chapter 10, Optimizing Images, by Stoyan Stefanov and Nicole Sullivan

These authors are experts in each of these areas. I wanted you to hear from them

directly, in their own voices. To help identify these chapters, the name(s) of the con-

tributing author(s) are on the chapter’s opening page.

JavaScript Performance

In my work analyzing today’s web sites, I consistently see that JavaScript is the key to

better-performing web applications, so I’ve started the book with these chapters.

Douglas Crockford wrote Chapter 1, Understanding Ajax Performance. Doug describes

how Ajax changes the way browsers and servers interact, and how web developers need

to understand this new relationship to properly identify opportunities for improving

performance.

Chapter 2, Creating Responsive Web Applications, by Ben Galbraith and Dion Almaer,

ties JavaScript performance back to what really matters: the user experience. Today’s

web applications invoke complex functions at the click of a button and must be eval-

uated on the basis of what they’re forcing the browser to do. The web applications that

succeed will be written by developers who understand the effects of their code on

response time.

I wrote the next four chapters. They focus on the mechanics of JavaScript—the best

way to package it and load it, and where to insert it in your pages. Chapter 3, Splitting

the Initial Payload, describes the situation facing many web applications today: a huge

JavaScript download at the beginning of the page that blocks rendering as well as further

downloads. The key is to break apart this monolithic JavaScript for more efficient

Chapters 4 and 5 go together. In today’s most popular browsers, external scripts block

everything else in the page. Chapter 4, Loading Scripts Without Blocking, explains how

to avoid these pitfalls when loading external scripts. Loading scripts asynchronously

presents a challenge when inlined code depends on them. Luckily, there are several

techniques for coupling inlined code with the asynchronous scripts on which they de-

pend. These techniques are presented in Chapter 5, Coupling Asynchronous Scripts.

Preface | xv

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Chapter 6, Positioning Inline Scripts, presents performance best practices that apply to

inline scripts, especially the impact they have on blocking parallel downloads.

I think of Chapter 7, Writing Efficient JavaScript, written by Nicholas C. Zakas, as the

complement to Doug’s chapter (Chapter 1). Whereas Doug describes the Ajax land-

scape, Nicholas zooms in on several specific techniques for speeding up JavaScript.

Network Performance

Web applications aren’t desktop applications—they have to be downloaded over the

Internet each time they are used. The adoption of Ajax has resulted in a new style of

data communication between servers and clients. Some of the biggest opportunities for

growth in the web industry are in emerging markets where Internet connectivity is a

challenge, to put it mildly. All of these factors highlight the need for improved network

performance.

In Chapter 8, Scaling with Comet, Dylan Schiemann describes an architecture that goes

beyond Ajax to provide high-volume, low-latency communication for real-time appli-

cations such as chat and document collaboration.

Chapter 9, Going Beyond Gzipping, describes how turning on compression isn’t enough

to guarantee optimal delivery of your web site’s content. Tony Gentilcore reveals a

little-known phenomenon that severely hinders the network performance of 15% of

the world’s Internet users.

Stoyan Stefanov and Nicole Sullivan team up to contribute Chapter 10, Optimizing

Images. This is a thorough treatment of the topic. This chapter reviews all popular

image formats, presents numerous image optimization techniques, and describes the

image compression tools of choice.

The remaining chapters were written by me. Chapter 11, Sharding Dominant Do-

mains, reminds us of the connection limits in the popular browsers of today, as well as

the next generation of browsers. It includes techniques for successfully splitting

resources across multiple domains.

Chapter 12, Flushing the Document Early, walks through the benefits and many gotchas

of using chunked encoding to start rendering the page even before the full HTML

document has arrived.

Browser Performance

Iframes are an easy and frequently used technique for embedding third-party content

in a web page. But they come with a cost. Chapter 13, Using Iframes Sparingly, explains

the downsides of iframes and offers a few alternatives.

Chapter 14, Simplifying CSS Selectors, presents the theories about how complex selec-

tors can impact performance, and then does an objective analysis to pinpoint the

situations that are of most concern.

xvi | Preface

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To comment or ask technical questions about this book, send email to:

bookquestions@oreilly.com

For more information about our books, conferences, Resource Centers, and the

O’Reilly Network, see our web site at:

http://www.oreilly.com

Using Code Examples

You may use the code in this book 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 this

book does require permission. Answering a question by citing this book and quoting

example code does not require permission. Incorporating a significant amount of ex-

ample 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: “Even Faster Web Sites, by Steve Souders.

If you feel your use of code examples falls outside fair use or the permission given above,

feel free to contact us at permissions@oreilly.com.

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Acknowledgments

I first want to thank the contributing authors: Dion Almaer, Doug Crockford, Ben

Galbraith, Tony Gentilcore, Dylan Schiemann, Stoyan Stefanov, Nicole Sullivan, and

Nicholas Zakas. They’ve made this a special book. Each of them is an expert in his or

her own right. Most of them have written their own books. By sharing their expertise,

they’ve helped create something unique.

xviii | Preface

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