Leave out elements unavailable, except replace a date with (n.d.) For no author, use title of webpage in that place. Use corporate author or government agency name, if available, instead of a single author.
If the author is the same as the website, omit the website component. If a page is continuously updated and does not have an archived version, include the retrieval date. Author, A. A. (Year). Title of page. Website. http://xxxxx
Corcodilos, N. (n.d.). Keep your salary under wraps. Ask the Headhunter. http://www.asktheheadhunter.com/hasalary.htm
U.S. Department of Veterans Affairs. (2010). Facing down PTSD, vet is now soaring high. http://www.mentalhealth.va.gov/featureArticle_Feb.asp
Online Encyclopedia (without print equivalent, not from an online database)
United States Holocaust Memorial Museum. (n.d.) Antisemitism. In Holocaust encyclopedia. Retrieved October 7, 2019, from https://encyclopedia.ushmm.org/content/en/article/antisemitism