A URL won't validate or returns "Unknown"
Marked's link validation uses a basic HEAD request to determine whether a link is valid. Anything other than a 200 (success) result will give either an unknown, or an error if it's a common error code such as 404 (not found) or 500 (server error). URLs behind authentication (such as Apple Developer urls or anything that requires login to access) will return an "unknown", as will certain sites such as Amazon.com where the server returns bizarre response codes. There's not much Marked can do about this.