This is a list of IDs and classes provided by MediaWiki core or a bundled extension/skin.
This are commonly used by extensions and gadget developers alike.
When in doubt, consider checking with Codesearch and Global Search to emperically understand usage.
For a more extended collection of historically grown list of IDs and classes, visit Wikipedia's Catalogue of CSS classes.
Set on text that is a comment (also known as edit summary or log reason), which appear in places such as page history, logs, and recent changes.
Set on the element containing the difference comparison between revisions and the action links for those revisions.
Set on the parent element containing the edit section link in section headings when viewing content. Added in MW 1.22 (change 49364).
The page title.
Set for the entire HTML document. This indicates the page was served through a MediaWiki skin. Commonly used to decide whether a style should not (or should only) apply to action=render and other external rendering contexts, e.g. in TemplateStyles or site styles.
Holds the content of the wiki page, or interface of the page action or special page.
Added on the same div as #mw-content-text, except on special pages and file pages, or when not viewing the page (e.g. history) or if the page doesn't exist. More info: directionality support. Added in MW 1.18.
Holds everything between the page title and contentSub on the one hand, and ArticleFeedback and categories on the other hand. Present on each page view (includes history view, non-existing pages, print view, ...). Known issue: May contain unrelated elements (bug 35247). Added in MW 1.19 (r111647).
#p-logo, #p-personal, #p-views, etc
identifiers for "Portlets" the skin may arrange in its navigation or sidebar. Useful for e.g. addPortletLink()
Used by MobileFrontend and Minerva to apply responsive sizing. As this can break some images and elements with absolute positioning (for overlays for instance), you can wrap images in an element with the class noresize to opt out from responsive sizing.
MobileFrontend and the mobile apps assume this is an infobox and will move it to after the lead of the article when they see fit. As well as applying dedicated styling for this often crucial element of Wikipedia articles.