Doing so helped one Google site that Souders declined to name speed up its initial page rendering by 60 percent.
Also, users tend to stay on certain sites, such as their Web mail, all day. These sites will re-render constantly throughout the day, incurring a delay from over-elaborate CSS files each time, Souders said.
"When I look at it, I feel like the teacher who hands out very severe grades," he said. Search engines with minimal content on the page, such as Google.com and Microsoft's Live.com, are among the rare sites that get an A from Yslow.
All these tools judge Web site performance by a set of rules, though none of them matches YSlow's 22 criteria.