The <source> element is used as a child of a <picture>, <audio>, or <video> element, and identifies the URL of one or more media resources. The <source> element is commonly used to add media resources in multiple formats for the best possible cross-browser compatibility.
The <section> element is a structural HTML element used to group together related elements. Each <section> typically includes one or more heading elements and additional elements presenting related content.
The <progress> element is used to create a progress bar to serve as a visual demonstration of progress towards the completion of task or goal. The max and value attributes are used to define how much progress (value) has been made towards task completion (max).
The <mark> element is used to highlight text inside of another element such as a paragraph, list, or table. Text to which the <mark> element has been added is considered to be particularly relevant in a specific context.
The <main> element is used to denote the content of a webpage that relates to the central topic of that page or application. It should include content that is unique to that page and should not include content that is duplicated across multiple webpages, such as headers, footers, and primary navigation elements.
The <xmp> element was used to surround HTML example text that should be rendered without interpreting any HTML elements between the opening and closing <xmp> tags. The element was deprecated in HTML 3.2 and is now obsolete.
The <noembed> element was conceived as a way to provide fallback content to users whose browsers did not support the <embed> element. In HTML 4.02 <noembed> was deprecated in favor of <object> to provide fallback content.
The <menu> element defines an instance of a menu. This experimental HTML feature has very limited browser support, but may soon be an effective way to add menu items to context menus and to create interactive web application menus.
The <small> element identifies text to display one size smaller than the surrounding text. In HTML5 the element is intended to be used to identify items of secondary importance such as copyright notices, side comments, and legal notices.