The <listing> element was intended as a way to render HTML code on a page. It was never properly supported, and is now deprecated. Using <listing> will almost certainly result in unexpected results. Instead, use <code>, or place the content in a <div> with the appropriate CSS styling.
The <nobr> element identifies text that should not be allowed to break into multiple lines which can force users to scroll horizontally to view the content. This element is obsolete and should be used.
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.
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 <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 <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 <select> element, used along with one or more <option> elements, creates a drop-down list of options for a web form. The <select> element creates the list and each <option> element is displayed as an available option in the list.
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 This element is obsolete.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.
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 This element is obsolete and should not be used.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 <meta> element is used to add machine-readable information to an HTML document. Information added with the <meta> tag is not displayed to website visitors but is provided for use by browsers and web crawlers.