When you cite a Web page, you can only cross your fingers and hope that the page will still be there, and still be the same, when your readers follow the URL.
WebCite may be the answer. They’ll archive whatever page you want, and give you a link to the archived page. It’s free.
Then do we all cross our fingers and hope that WebCite will still be there?
I don’t, but you can if you want.
For a longer, more boring, answer:
“WebCite® is a member of the International Internet Preservation Consortium (IIPC). Members of the IIPC (e.g. libraries) are concerned with the collection, preservation and long-term access of a rich body of Internet content from around the world, and develop and use common tools, techniques and standards for the creation of international archives, which may also include the exchange of data. WebCite® feeds its content to digital preservation partners such as libraries and the Internet Archive (archive.org). WebCite® is operated and supported by publishers, who are already using it for their journals and citations and therefore have a vital interest in keeping the service alive.
Finally, the DOI handle system enables a mechanism to cite and retrieve an archived copy of a webpage without having to rely on the functionality of WebCite® and the webcitation.org URL itself. WebCite® can, under certain circumstances, assign DOIs to archived copies, which are identified by their hash (a digital fingerprint). Archived copies can then be retrieved through http://dx.doi.org (either from WebCite®, or from other digital preservation organizations), providing a mechanism for cross-archive retrieval of archived material.”