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High Performance Web Sites

Cover of High Performance Web Sites by Steve Souders Published by O'Reilly Media, Inc.
  1. High Performance Web Sites
    1. SPECIAL OFFER: Upgrade this ebook with O’Reilly
    2. Praise for High Performance Web Sites
    3. Foreword
    4. Preface
      1. How This Book Is Organized
      2. Conventions Used in This Book
      3. Code Examples
      4. Comments and Questions
      5. Safari® Books Online
      6. Acknowledgments
    5. 1. The Importance of Frontend Performance
      1. Tracking Web Page Performance
      2. Where Does the Time Go?
      3. The Performance Golden Rule
    6. 2. HTTP Overview
      1. Compression
      2. Conditional GET Requests
      3. Expires
      4. Keep-Alive
      5. There's More
    7. 3. Rule 1: Make Fewer HTTP Requests
      1. Image Maps
      2. CSS Sprites
      3. Inline Images
      4. Combined Scripts and Stylesheets
      5. Conclusion
    8. 4. Rule 2: Use a Content Delivery Network
      1. Content Delivery Networks
      2. The Savings
    9. 5. Rule 3: Add an Expires Header
      1. Expires Header
      2. Max-Age and mod_expires
      3. Empty Cache vs. Primed Cache
      4. More Than Just Images
      5. Revving Filenames
      6. Examples
    10. 6. Rule 4: Gzip Components
      1. How Compression Works
      2. What to Compress
      3. The Savings
      4. Configuration
      5. Proxy Caching
      6. Edge Cases
      7. Gzip in Action
    11. 7. Rule 5: Put Stylesheets at the Top
      1. Progressive Rendering
      2. sleep.cgi
      3. Blank White Screen
      4. Flash of Unstyled Content
      5. What's a Frontend Engineer to Do?
    12. 8. Rule 6: Put Scripts at the Bottom
      1. Problems with Scripts
      2. Parallel Downloads
      3. Scripts Block Downloads
      4. Worst Case: Scripts at the Top
      5. Best Case: Scripts at the Bottom
      6. Putting It in Perspective
    13. 9. Rule 7: Avoid CSS Expressions
      1. Updating Expressions
      2. Working Around the Problem
      3. Conclusion
    14. 10. Rule 8: Make JavaScript and CSS External
      1. Inline vs. External
      2. Typical Results in the Field
      3. Home Pages
      4. The Best of Both Worlds
    15. 11. Rule 9: Reduce DNS Lookups
      1. DNS Caching and TTLs
      2. The Browser's Perspective
      3. Reducing DNS Lookups
    16. 12. Rule 10: Minify JavaScript
      1. Minification
      2. Obfuscation
      3. The Savings
      4. Examples
      5. Icing on the Cake
    17. 13. Rule 11: Avoid Redirects
      1. Types of Redirects
      2. How Redirects Hurt Performance
      3. Alternatives to Redirects
    18. 14. Rule 12: Remove Duplicate Scripts
      1. Duplicate Scripts—They Happen
      2. Duplicate Scripts Hurt Performance
      3. Avoiding Duplicate Scripts
    19. 15. Rule 13: Configure ETags
      1. What's an ETag?
      2. The Problem with ETags
      3. ETags: Use 'Em or Lose 'Em
      4. ETags in the Real World
    20. 16. Rule 14: Make Ajax Cacheable
      1. Web 2.0, DHTML, and Ajax
      2. Asynchronous = Instantaneous?
      3. Optimizing Ajax Requests
      4. Caching Ajax in the Real World
    21. 17. Deconstructing 10 Top Sites
      1. Page Weight, Response Time, YSlow Grade
      2. How the Tests Were Done
      3. Amazon
      4. AOL
      5. CNN
      6. eBay
      7. Google
      8. MSN
      9. MySpace
      10. Wikipedia
      11. Yahoo!
      12. YouTube
    22. Index
    23. About the Author
    24. Colophon
    25. SPECIAL OFFER: Upgrade this ebook with O’Reilly
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Yahoo!

Figure 17-23. http://www.yahoo.com

Yahoo! (http://www.yahoo.com) is the fourth-heaviest page in total bytes among the ones examined in this chapter, but second in response time and YSlow grade. The Yahoo! home page team has been engaged with my performance team for years, and is constantly tracking and working to improve response times. As a result, their YSlow scores are high, and they are able to squeeze more speed out of their page.

Yahoo!'s home page has four CSS sprite images. It has been using sprites for years and was the first web site in which I encountered the use of sprites. One of these sprites is icons_1.5.gif. Looking at the list of components, we see that this image is downloaded twice. On further investigation, the issue is that two URLs reference the exact same image:

http://us.js2.yimg.com/us.js.yimg.com/i/ww/sp/icons_1.5.gif
http://us.i1.yimg.com/us.yimg.com/i/ww/sp/icons_1.5.gif

How does a mistake like this happen? Template variables are most likely used to build these URLs. The CSS rules that include this background image are both inlined in the HTML document, so presumably both had access to the same template variables. The us.js2.yimg.com hostname is used for all of the scripts, and us.i1.yimg.com is used solely for images and Flash. Most likely, the "JavaScript" hostname, us.js2.yimg.com, was accidentally used for this CSS background image.

This look at the ...

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