I was working in Liferay's Knowledge Base with content that includes the HTML 5 aside tag. The Content field in this application has a portlet model hint that calls the sanitizer. Whenever I'd use an aside tag (a legitimate HTML 5 tag), the sanitizer would strip it out.
Liferay's sanitizer needs to be updated so that legitimate HTML 5 tags can be used.
HTML5 new elements: https://www.w3schools.com/hTML/html5_new_elements.asp
Old elements not in list: address