When referencing an external resource in a publication, see if there is a version of the resource that has a unique persistent identifier and if so use that identifier to reference it. While all "persistent" identifiers can eventually break depending on whether they are properly maintained, they are more likely to last than other links and uniquely identify a resource. Another option for tackling “link rot” - the term for when links stop working - is to use a web archiving snapshot service such as archive.today or Internet Archive’s Save Page Now service to archive the page and reference the resulting snapshot as an alternative link in the document. Robust Links are one way to present this to users.
These guidelines cover other instances that may benefit from use of identifiers:
27. Assign persistent identifiers to publication resources and use them
31. Assign identifiers to significant new versions of the work