URIs which it passes to an external handler (e.g. mailto:), it only needs to identify the scheme (to select the correct handler); it is the handler's responsibility to validate its own URIs (i.e. mail programs need to validate mailto: URIs).
I don't agree. Whatever program takes input from an untrusted source, it's that programs duty to sanitize the input before passing it on to internal components. It's like a firewall, you filter before it gets inside the system.
Example, an ftp server has to sanitize filenames to prevent useage of streams on NTFS, you don't blame the filesystem that the input gets passed to, it's the job of the ftp server to do the sanitizing of untrusted input.
Geo.