Manual:Hooks/OutputPageBodyAttributes

Details
Allows extensions to add attributes to the body of the page they might need. Or to allow building extensions to add body classes that aren't of high enough demand to be included in core.
 * $out: The OutputPage which called the hook, can be used to get the real title
 * $sk: The Skin that called OutputPage::headElement
 * &$bodyAttrs: An array of attributes for the body tag passed to Html::openElement