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 <sound> element was used in the Mosaic browser to add sound files to HTML documents. It was never formally adopted in an HTML standard and has no support among modern browsers. Instead, use the modern HTML5 element <audio>.
The <link> element is used to define a relationship between an HTML document and an external resource. This element is most commonly used to define the relationship between a document and one or more external CSS stylesheets.
The <plaintext> element was used to render HTML code as plain text. Since everything after the opening tag was rendered as plain text, there was no closing <plaintext> tag. This element is obsolete and should not be used. Instead, use the <code> or <pre> elements.
The <spacer> element was used to add whitespace to web pages by replicating the effect of adding a small image to web page of the same color as the background color of the web page. This element is obsolete. Use modern CSS equivalents to add space between elements of a web page.