Nesting of browsing context mostly happens with <iframe> and <object>. Thus an HTML web page will most of the time be a "top-level" document and all its <iframe> and <object> (recursively).
Although web pages as defined here are sets of documents (and do not contain other kind of nodes), one can abusively write that any node is "in a web page" if it is a shadow-including descendant of a document that is part of that web page.