I've always been curious why certain characters (e.g. commas) haven't been allowed in the url. I've never seen one used in a specialized way at all, are they just nonexistent in usage? If so, why?
My best guess. From RFC 3986 (link below)<p><i>For example, the semicolon (";") and equals ("=") reserved characters are
often used to delimit parameters and parameter values applicable to
that segment. The comma (",") reserved character is often used for
similar purposes. For example, one URI producer might use a segment
such as "name;v=1.1" to indicate a reference to version 1.1 of
"name", whereas another might use a segment such as "name,1.1" to
indicate the same.</i><p><a href="http://www.ietf.org/rfc/rfc3986.txt" rel="nofollow">http://www.ietf.org/rfc/rfc3986.txt</a><p>EDIT: This RFC is for URIs and your question was about URLs. URLs are generally considered a subset of URIs, but according to this SO answer, that might be open to debate. <a href="http://stackoverflow.com/questions/176264/whats-the-difference-between-a-uri-and-a-url" rel="nofollow">http://stackoverflow.com/questions/176264/whats-the-differen...</a>
Vignette StoryServer, a CMS from back in the day, had commas in all (or almost all) the URLs it generated.<p><a href="http://en.wikipedia.org/wiki/StoryServer" rel="nofollow">http://en.wikipedia.org/wiki/StoryServer</a>