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136 | Chapter 15: Deconstructing 10 Top Sites Figure 15-26. YouTube HTTP requests html stylesheet stylesheet script image script script script script html image image DNS lookup image image image image image DNS lookup image image script image image image image image image image image image DNS lookup image image script image image image image image image image image image image image image image image image image image image image image image image image image image image image

• 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