Since T37247 there is a new <div> with the class "mw-parser-output". This is the innermost container for the page content.
In HTML5, there is a new <main> tag. As can be seen at https://developer.mozilla.org/en-US/docs/Web/HTML/Element/main, the <main> tag is for the main content, excluding any sidebars, navigation links, copyright information, site logos or other UI elements.
This <div> is defined in https://phabricator.wikimedia.org/source/mediawiki/browse/master/includes/parser/Parser.php.