Wikipedia:Shallow references
 | This is an information page. It is not an encyclopedic article, nor one of Wikipedia's policies or guidelines; rather, its purpose is to explain certain aspects of Wikipedia's norms, customs, technicalities, or practices. It may reflect differing levels of consensus and vetting. |
 | This page in a nutshell: Link directly to documents cited, not to higher-level pages on websites where they are hosted. |
Per consensus at Wikipedia:Village pump (policy)/Archive 102#Prohibited (sic) links in January 2013:
When citing a reference, with a URL, the URL should link to the document (web page) cited, not just the home page (or some other high level page) on the site which hosts that document.