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YSlow is a Firefox add-on integrated with the popular Firebug web development tool from the Yahoo!’s Exceptional Performance team which provides improved performance measurement tools and optimization advice.

YSlow analyzes the page’s components, including downloaded components, using advanced techniques and tells you why they’re slow based on Yahoo’s 13 identified basic rules for high performance web sites.

To run YSlow, select Performance, Stats, or Components, or right-click YSlow in the status bar. When YSlow analyzes the page, the Firebug window displays the Performance view, and the status bar at the bottom of the window displays the overall grade for the page, and its size.

How YSlow works:

1. YSlow crawls the DOM to find all the components (images, scripts, stylesheets, etc.) in the page.
2. YSlow gets information about each component: size, whether it was gzipped, Expires header, etc. YSlow gets this information from Firebug’s Net panel if it’s available. If the component’s information is not available from Net panel (for example, the component was read from cache or it had a 304 response) YSlow makes an XMLHttpRequest to fetch the component and track its headers and other necessary information.
3. YSlow takes all this data about the page and generates a grade for each rule, which produces the overall grade.

YSlow gives you:

• Performance report card
• HTTP/HTML summary
• List of components in the page
• Tools including JSLint

With YSlow, you can:

• Build faster web pages
• Reduce your end-user response times
• Get the most potential for improvement by focusing on the front-end

YSlow also allows you to view results from the Performance, Stats, and Components views in printer friendly format.

You must install Firebug in order to use YSlow. Once Firebug is installed, download YSlow from http://developer.yahoo.com/yslow and follow the prompts to install it. Firebug must be enabled, and the Firebug window must be open to run YSlow and view the results. For more on how to use and customize YSlow, visit the Help page.

YSlow is licensed under the Mozilla Public License (MPL) version 1.1, with portions licensed by third parties under other license terms. To review the terms before installing, see License.