Common.js and Common.css are pages which influence the interface and layout of Wikipedia.

Common.js may refer to one of several JavaScript pages:

• Wikipedia's sitewide "common JavaScript" page, located at MediaWiki:Common.js. This is loaded for all users.
• An individual user's "common JavaScript" page, located at Special:MyPage/common.js. Unlike Special:MyPage/skin.js, which redirects to the JavaScript page of the user's current skin, a user's common.js page is loaded regardless of which skin they are using.

Similarly,

Common.css may refer to one of several Cascading Style Sheets (CSS) pages:

• Wikipedia's sitewide "common CSS" page, located at MediaWiki:Common.css. This is loaded for all users.
• An individual user's "common CSS" page, located at Special:MyPage/common.css. Unlike Special:MyPage/skin.css, which redirects to the CSS page of the user's current skin, a user's common.css page is loaded regardless of which skin they are using.

• the MediaWiki interface manual, especially the subpages on JavaScript and Stylesheets
• Explanation how a user can customize the skin with these pages
• Overview of these and related pages
• Use of CSS