There is no <ISINPUT> element, just <ISINDEX>. ;-)

Doc/lib/libcgi.tex

@@ -22,7 +22,7 @@ written in Python.
 \nodename{cgi-intro}
 
 A CGI script is invoked by an HTTP server, usually to process user
-input submitted through an HTML \code{<FORM>} or \code{<ISINPUT>} element.
+input submitted through an HTML \code{<FORM>} or \code{<ISINDEX>} element.
 
 Most often, CGI scripts live in the server's special \file{cgi-bin}
 directory.  The HTTP server places all sorts of information about the