User blog comment:Rappy 4187/Technical Update: May 11, 2016/@comment-11733175-20160511202337/@comment-126761-20160511203559

In many cases it would simply match the URL - e.g. runescape.wikia.com's database is 'runescape'. The CSS class is then 'wiki-runescape'.

International communities generally just have the language prefix at the start, such as 'derunescape'.