From PURDUE OWL:
Optional elements:
The eighth edition is designed to be as streamlined as possible. The author should include any information that helps readers easily identify the source, without including unnecessary information that may be distracting. The following is a list of select optional elements that should be part of a documented source at the writer’s discretion.
Date of original publication:
If a source has been published on more than one date, the writer may want to include both dates if it will provide the reader with necessary or helpful information.
Erdrich, Louise. Love Medicine. 1984. Perennial-Harper, 1993.
City of publication:
The seventh edition handbook required the city in which a publisher is located, but the eighth edition states that this is only necessary in particular instances, such as in a work published before 1900. Since pre-1900 works were usually associated with the city in which they were published, your documentation may substitute the city name for the publisher’s name.
Thoreau, Henry David. Excursions. Boston, 1863.
Date of access:
When you cite an online source, the MLA Handbook recommends including a date of access on which you accessed the material, since an online work may change or move at any time.
Bernstein, Mark. "10 Tips on Writing the Living Web." A List Apart: For People Who Make Websites, 16 Aug. 2002, alistapart.com/article/writeliving. Accessed 4 May 2009.
URLs:
As mentioned above, while the eighth edition recommends including URLs when you cite online sources, you should always check with your instructor or editor and include URLs at their discretion.
DOIs:
A DOI, or digital object identifier, is a series of digits and letters that leads to the location of an online source. Articles in journals are often assigned DOIs to ensure that the source is locatable, even if the URL changes. If your source is listed with a DOI, use that instead of a URL.
Alonso, Alvaro, and Julio A. Camargo. "Toxicity of Nitrite to Three Species of Freshwater Invertebrates." Environmental Toxicology, vol. 21, no. 1, 3 Feb. 2006, pp. 90-94. Wiley Online Library, doi: 10.1002/tox.20155.