Link rot is an informal term used to describe hyperlinks that no longer point to content on a website, resulting in broken links. This can occur when a linked site's content is moved or removed.
Avoid Deep Linking: try to link to the highest reasonable level of a website and describe the site well enough that should the link need updating one could easily use an internet search to find the correct page. If a specific document is needed, you can provide both a link to the document and also within the description box link to the top level of the website.
Links in References
References refer to material that was available at the time that the document was edited. Consider linking to cached version of a document via a service such as the Internet Archive or Archive.is or include a DOI or, if available, link to the relevant WebCite reference (for some academic sources). Always include information about the document so that a reader interested in finding the source can do so, even if the link is no longer available.