All of us have experienced the dreaded “Error 404 – File Not Found” message while surfing the web. Charlie Priore, our former Science Librarian, estimates that the average lifetime of information on the web is about 3 years. Contrast that with your opportunity to walk to Gould Library and browse through any copy of Physical Review for the last 106 years and read the same text that Michelson, Millikan, Einstein, Compton, Fermi, … read.
Because of the ethereal nature of web information, citations to the web are generally discouraged in scholarly work. We won’t set a strict limit on such citations, since the appropriateness will depend on the subject to some extent. But in general you should think in terms of “Few” (two or three) rather than “any.” Instead, track down and cite the original literature behind the web information. When you do cite the web, try to include all of this information: author, date posted, title, and date accessed.