Stefan Hayden wrote in iCal technology and webcal protocol and their lack of support:
Another feature to bootstrap the iCal technology is a (new?) web protocol called webcal. With this you replace http:// with webcal:// and this gets your browser to launch your iCal enabled calendar to add the event. I don’t know much about webcal since there is little written about the protocol. While Firefox recognises the protocol IE has no idea what it is and just refuses to do anything. It reminds me of another protocol where you replace http:// with feed:// to denote a xml feed but even my up-to-date firefox doesn’t know what to do with that and I don’t know exactly what program does. Even Wikipedia doesn’t seem to have anything to say about either feed:// or webcal://.
Here are some answers:
The Official Uniform Resource Identifier (URI) SCHEMES, such as, HTTP and FTP, are registered with IANA. Webcal was initiated by Apple for use with the Apple iCal application and has become the common standard for accessing calendaring information via WebDAV. Apple has never registered the protocol with IANA, so browsers and calendaring applications that support the webcal protocol are just making life simpler for users accessing the thousands of calendar files posted on the Internet.
Other Calendaring Protocols:
CalDAV is planned to be the offical protocol allowing calendar access via WebDAV. However, the CalDAV draft does not call for using webcal:// or CalDav://. Instead it calls for using Http:// using the HTTP Option feature to indicate the information is in a CalDAV format.
The Calendar Access Protocol (CAP) has been approved and CAP:// will be the Official Uniform Resource Identifier registered with IANA. While approved, the process to receive the RFC number and IANA registration takes times. CAP specifies how to query, create, modify, and delete iCalendar components over the wire.