In operation, YSlow uses either one of three predefined rulesets, or a user-defined ruleset, to grade web pages; offering suggestions on how to improve that page's performance. The report also summarizes the page's components and displays statistics about the page.
YSlow is integrated with the Firebug web development tool, which is required for its use. Firebug allows users to edit, debug, and monitor CSS, HTML, and JavaScript in realtime and on any web page. It is intended by its creators "to put a wealth of development tools at your fingertips while you browse."
YSlow also incorporates Yahoo's free Smush.it image optimization service, which is no longer available as a separate download.
"Image optimization is an art few have mastered," states the Smush.it website. "Useful image editing tools exist that allow images to be edited to reduce their file size while retaining image quality. Using these tools is a good start for optimizing images, but more can be done using advanced tools like Smush.it."
According to Yahoo, its Smush.it service uses optimization techniques specific to the particular image format to remove unnecessary bytes from image files.
"It is a 'lossless' tool, which means it optimizes the images without changing their look or visual quality," the Smush.it site states. "After Smush.it runs on a web page it reports how many bytes would be saved by optimizing the page's images and provides a downloadable zip file with the minimized image files."
Adult operators should note that the Smush.it TOS states that "You will not include any images with content that is unlawful, harmful, threatening, abusive, harassing, tortious, defamatory, vulgar, pornographic, offensive, indecent, libelous, invasive of another's privacy, hateful, or racially, ethnically or otherwise objectionable;" so explicit content may be prohibited.
