Heading elements implement six levels of document headings, <h1> is the most important and <h6> is the least. A heading element briefly describes the topic of the section it introduces. Heading information may be used by user agents, for example, to construct a table of contents for a document automatically.
The HTML <hr> element represents a thematic break between paragraph-level elements (for example, a change of scene in a story, or a shift of topic with a section). In previous versions of HTML, it represented a horizontal rule. It may still be displayed as a horizontal rule in visual browsers, but is now defined in semantic terms, rather than presentational terms.
The HTML Inline Frame Element (<iframe>) represents a nested browsing context, effectively embedding another HTML page into the current page. In HTML 4.01, a document may contain a head and a body or a head and a frameset, but not both a body and a frameset. However, an <iframe> can be used within a normal document body. Each browsing context has its own session history and active document. The browsing context that contains the embedded content is called the parent browsing context. The top-level browsing context (which has no parent) is typically the browser window.
<isindex> is an obsolete HTML element that puts a text field in a page for querying the document.<isindex> was providing a single line text input for entering a query string. When sent, the server would return a list of pages matching the query. It supports depended on both the browser and the server to react to the query.
The HTML Label Element (<label>) represents a caption for an item in a user interface. It can be associated with a control either by placing the control element inside the <label> element, or by using the for attribute. Such a control is called the labeled control of the label element. One input can be associated with multiple labels.
The HTML <li> element (or HTML List Item Element) is used to represent an item in a list. It must be contained in a parent element: an ordered list (<ol>), an unordered list (<ul>), or a menu (<menu>). In menus and unordered lists, list items are usually displayed using bullet points. In ordered lists, they are usually displayed with an ascending counter on the left, such as a number or letter.
The HTML Listing Element (<listing>) renders text between the start and end tags without interpreting the HTML in between and using a monospaced font. The HTML 2 standard recommended that lines shouldn't be broken when not greater than 132 characters.
The HTML <main> element represents the main content of the <body> of a document or application. The main content area consists of content that is directly related to, or expands upon the central topic of a document or the central functionality of an application. This content should be unique to the document, excluding any content that is repeated across a set of documents such as sidebars, navigation links, copyright information, site logos, and search forms (unless the document's main function is as a search form).
<nextid> is an obsolete HTML element that served to enable the NeXT web designing tool to generate automatic NAME labels for its anchors. It was generated by that web editing tool automatically and was not to be adjusted or entered by hand. This element has the distinction of being the first element to become one of the "Lost Tags" by being eliminated from the official public DTD's of the HTML versions. It is also probably one of the least understood of all of the early HTML elements.