If you validate a Pinecast feed with the W3C validator, you'll find that it produces some errors.
None of these are problems, and all of them stem from issues with the W3C validator service.
What does "href must be a full URL" mean?
This error is happens because the W3C validator does not understand HTTPS URLs (we've tested without HTTPS and the errors go away). HTTPS is important because it ensures security for your listeners and allows your feed to continue supporting new versions of Apple Podcasts.
What does "Undefined item element dc:copyright" mean?
Pinecast uses a Dublin Core extension to RSS to add extra information like copyright to your feed. RSS allows for these extensions (with podcast apps that don't support them simply ignoring the metadata), but the W3C's validator isn't built with this understanding.