The code for Product Links includes the iframe tag which is not XHMTL 1.0 Strict. As a result, pages with Amazon ads fail validation on the default Drupal Install.

The iframe tag, however, is valid XHTML 1.0 "Transitional", so one only need change the document type from 1.0 Strict to 1.0 Transitional. Thereafter, pages with Amazon ads will pass validation.

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