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Web Performance Daybook Volume 2 by Stoyan Stefanov

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Chapter 9. All about YSlow

Betty Tso

Since 2007, millions of developers have been using YSlow to help them find out ways to make their web pages load faster. YSlow score has been the standard for Performance measurement in dev, QA, and production stages.

YSlow first started as a bookmarklet by Steve Souders while at Yahoo!, and soon became a popular Firefox extension. Over the past year, Marcel Duran built a YSlow Chrome extension, Opera extension, and Safari extension. In order to also support mobile devices as well as other browsers, YSlow was also made available as a bookmarklet in June 2011 with fresh shiny code and new architecture.

While speaking at Velocity China on December 7, 2011, our team announced the release of YSlow for Command Line beta, with courtesy to our FE tech lead, Marcel. This version leverages Node.js and takes .har files as input to generate YSlow score for a URL. Several output options are available—JSON, XML, and plain text. Users can also pipe the result to a beacon server, such as http://www.showslow.com/beacon/yslow/ and view the result in a graphical UI. For complete YSlow beacon spec, refer to the users’ guide.

In February 2012, YSlow was open sourced on Github and given a new home: yslow.org. Since then, YSlow has become a community-driven tool—within the first 24 hours of the open source announcement, there were 437 watchers and 37 forks.

While speaking at Amazon’s annual frontend conference in April 2012 (http://wh.yslow.org/amazon-wdc), Marcel Duran ...

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