The http://en.wikipedia.org/wiki/Neptune page is invalid XHTML, because some of the "id" attributes for <cite> elements are not unique.
STEPS TO REPRODUCE:
- Launch the validator: http://validator.w3.org/
- Enter the address: http://en.wikipedia.org/wiki/Neptune
- Click the "Check" button, or press the Return key.
- Errors found while checking this document as XHTML 1.0 Transitional!
- Result: 6 Errors
- Line 697, Column 128: ID "CITEREFPodolakWeizman.2C_A..3B_Marley.2C_M.1995" already defined.
- An "id" is a unique identifier...
- This document was successfully checked as XHTML 1.0 Transitional!
- Result: Passed
- In the wikitext, the <ref name=Podolak1995> and <ref name=pass43> elements contain the same information (except that the first also contains an url).
- The duplicate "id" attributes don't appear to be used within the XHTML document.
- Possibly related to Bug 10218 [Sanitizer doesn't validate attribute values according to DTD].