<podcast:alternateEnclosure> should not be allowed at the channel level without more work

[mattbasta]

The spec allows for <podcast:alternateEnclosure> to be present in the <channel>. While this is a fun idea, it has a number of challenging problems that deserve more attention before formally allowing it.

For one, the value isn't an "alternate" to anything. It's just an enclosure. Semantically, the tag makes no sense.

Some RSS extensions already support the channel-level case with other tags, which provide more prescriptive guidance on their use:

• RadioPublic has the <rp:gateway-episodes> and <rp:greatest-hits> elements, which reference the GUID of an <item> https://radiopublic.com/schema/1.0/#gateway
• Apple's spec allows for an epiosde type to be defined on an <item>, which allows a full <item> to describe trailers and bonus content. A more expressive variant of this would allow the show's content to be visible in a backwards-compatible way to all podcast apps, and make that content mobile between hosts that do not support this spec.