Use of extension attribute on RSS 2.0 core element:
While the original RSS 2.0 specification only explicitly allows arbitrary namespace qualified elements to be defined, it is silent on the permissibility of arbitrary namespace qualified attributes on core elements. In June of 2007, the RSS Advisory Board approved a clarification authorizing such use. Without stating a reason, Harvard had previously stopped accepting updates from the RSS Advisory Board.
Here are a few links to prior discussions on the topic:
As no known problems have arisen as of yet from the use of namespace qualified attributes, no action may be needed; however if there are equal or better alternatives (such as the use of sibling namespace qualified elements), you might consider using them instead.
Alternately, by explictly adopting the value of the
<docs> element specified in the RSS Advisory Board's version of the specification, you are indicating that you are relying on the specification and profile published by the RSS Advisory Board. If the Feed Validator detects such an usage, it will omit this warning message
Not clear? Disagree?
You might be able to find help in one of these fine resources.