How to use aside in HTML5
Initially one would consider the <aside> element to display a sidebar. The aside element could indeed be used for this specific purpose but besides it can also be used to mark other elements of a webpage. If, for example, the aside element is used within an <article> element, the parser will consider the aside element content to be part of the article element content. If on the other hand the aside element is not used within another element but forms part of the <body> of the document the parser will consider the aside element content as secondary content which doesn’t belong to a specific part of the webpage.