Citations from web pages:

Authors may sometimes wish to cite information available from the internet in similar ways to the citation of published literature. In using this option, authors are asked to ensure that:
fully authenticated addresses are included in the reference list, along with titles, years and authors of the sources being cited, and the most recent date the site was accessed; the sites or information sources have sufficient longevity and ease of access for others to follow up the citation; the information is of a scientific quality at least equal to that of peer-reviewed information available in learned scientific journals; hard literature sources are used in preference where they are available. It is likely that official web sites from organisations such as learned societies, government bodies or reputable NGOs will most often satisfy quality criteria.