The HTML <head> element provides general information (metadata) about the document, including its title and links to its scripts and style sheets.
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.
The HTML <img> element represents an image in the document.
The HTML <ins> Element (or HTML Inserted Text) HTML represents a range of text that has been added to a document.
<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 <legend> Element (or HTML Legend Field Element) represents a caption for the content of its parent <fieldset>.
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).
The HTML <meter> Element represents either a scalar value within a known range or a fractional value.
The HTML <nav> element (HTML Navigation Element) represents a section of a page that links to other pages or to parts within the page: a section with navigation links.
<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.
<noframes> is an HTML element which is used to supporting browsers which are not able to support <frame> elements or configured to do so.
The HTML <noscript> Element defines a section of html to be inserted if a script type on the page is unsupported or if scripting is currently turned off in the browser.
The HTML Embedded Object Element (<object>) represents an external resource, which can be treated as an image, a nested browsing context, or a resource to be handled by a plugin.
In a Web form, the HTML <optgroup> element  creates a grouping of options within a <select> element.
In a Web form, the HTML <option> element is used to create a control representing an item within a <select>, an <optgroup> or a <datalist> HTML5 element.