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116 | Chapter 15: Deconstructing 10 Top Sites eBay The YSlow grade for eBay (http://www.ebay.com) is very close to a B. With a little bit of work it would perform well. The main problems are with Rules 1, 3, 9, and 13. Figure 15-10. http://www.ebay.com YSlow grade Page weight HTTP requests Response time C 275K 62 9.6 sec

• Cover
• Table of Contents
• Foreword
• Preface
• How This Book Is Organized
• Conventions Used in This Book
• Code Examples
• Comments and Questions
• Safari® Books Online
• Acknowledgments
• The Importance of Frontend Performance
• Tracking Web Page Performance
• Where Does the Time Go?
• The Performance Golden Rule
• HTTP Overview
• Compression
• Conditional GET Requests
• Expires
• Keep-Alive
• There's More
• Rule 1: Make Fewer HTTP Requests
• Image Maps
• CSS Sprites
• Inline Images
• Combined Scripts and Stylesheets
• Conclusion
• Rule 2: Use a Content Delivery Network
• Content Delivery Networks
• The Savings
• Rule 3: Add an Expires Header
• Expires Header
• Max-Age and mod_expires
• Empty Cache vs. Primed Cache
• More Than Just Images
• Revving Filenames
• Examples
• Rule 4: Gzip Components
• How Compression Works
• What to Compress
• The Savings
• Configuration
• Apache 1.3: mod_gzip
• Apache 2.x: mod_deflate
• Proxy Caching
• Edge Cases
• Gzip in Action
• Rule 5: Put Stylesheets at the Top
• Progressive Rendering
• sleep.cgi
• Blank White Screen
• CSS at the Bottom
• CSS at the Top
• Flash of Unstyled Content
• What's a Frontend Engineer to Do?
• Rule 6: Put Scripts at the Bottom
• Problems with Scripts
• Parallel Downloads
• Scripts Block Downloads
• Worst Case: Scripts at the Top
• Best Case: Scripts at the Bottom
• Putting It in Perspective
• Rule 7: Avoid CSS Expressions
• Updating Expressions
• Working Around the Problem
• One-Time Expressions
• Event Handlers
• Conclusion
• Rule 8: Make JavaScript and CSS External
• Inline vs. External
• In Raw Terms, Inline Is Faster
• Page Views
• Empty Cache vs. Primed Cache
• Component Reuse
• Typical Results in the Field
• Home Pages
• The Best of Both Worlds
• Post-Onload Download
• Dynamic Inlining
• Rule 9: Reduce DNS Lookups
• DNS Caching and TTLs
• Factors Affecting DNS Caching
• TTL Values
• The Browser's Perspective
• Internet Explorer
• Firefox
• Reducing DNS Lookups
• Rule 10: Minify JavaScript
• Minification
• Obfuscation
• The Savings
• Examples
• Icing on the Cake
• Inline Scripts
• Gzip and Minification
• Minifying CSS
• Rule 11: Avoid Redirects
• Types of Redirects
• How Redirects Hurt Performance
• Alternatives to Redirects
• Missing Trailing Slash
• Connecting Web Sites
• Tracking Internal Traffic
• Tracking Outbound Traffic
• Prettier URLs
• Rule 12: Remove Duplicate Scripts
• Duplicate Scripts—They Happen
• Duplicate Scripts Hurt Performance
• Avoiding Duplicate Scripts
• Rule 13: Configure ETags
• What's an ETag?
• Expires Header
• Conditional GET Requests
• Last-Modified Date
• Entity Tags
• The Problem with ETags
• ETags: Use 'Em or Lose 'Em
• ETags in the Real World
• Rule 14: Make Ajax Cacheable
• Web 2.0, DHTML, and Ajax
• Web 2.0
• DHTML
• Ajax
• Asynchronous = Instantaneous?
• Optimizing Ajax Requests
• Caching Ajax in the Real World
• Yahoo! Mail
• Google Spreadsheets
• Deconstructing 10 Top Sites
• Page Weight, Response Time, YSlow Grade
• How the Tests Were Done
• Amazon
• AOL
• CNN
• eBay
• Google
• MSN
• MySpace
• Wikipedia
• Yahoo!
• YouTube
• Index