Web pages change, move, or disappear.
The persistent identifiers (PID) for documents are publisher's commitment to keep documents available regardless of changes in web pages. The persistent identifiers are registered centrally. When an end-user wishes to access a certain document, the identifier in his request is ‘resolved’, i.e. the correct document is retrieved, without the end-user needing to know the exact location of the document.
Persistent identifiers are the following:
Reconsider using web sources as references. Avoid reference rot, your references should be available long after publishing your work. Some high-risk examples: