YSlow: Avoid HTTP 404 (Not Found) error
Fixup any 404 errors to avoid your users fetching content that does not exist.
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Details from Yahoo!
Making an HTTP request and receiving a 404 (Not Found) error is expensive and degrades the user experience.
Some sites have helpful 404 messages (for example, "Did you mean ...?"), which may assist the user, but server resources are still wasted.