<frame> is an HTML element which defines a particular area in which another HTML document can be displayed. A frame should be used within a <frameset>.
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 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.
<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.
The <noembed> element is a deprecated and non-standard way to provide alternative, or "fallback", content for browsers that do not support the <embed> element or do not support embedded content an author wishes to use. This element was deprecated in HTML 4.01 and above in favor of <object>. Fallback content should be inserted between the opening and closing <object> tags.
The HTML <picture> element is a container used to specify multiple <source> elements for a specific <img> contained in it. The browser will choose the most suitable source according to the current layout of the page (the constraints of the box the image will appear in) and the device it will be displayed on (e.g. a normal or hiDPI device.)
<spacer> is an HTML element which is used for inserting white spaces to web pages. It was created by NetScape for achieving same effect as a single-pixel layout GIF image, which was something web designers used to use to add white spaces to web pages, without actually using a GIF. However <spacer> is not supported by any major browser and same effects can be created with various CSS rules. In Mozilla applications, support for this element was removed in Gecko 2.0. Therefore usage of <spacer> is unnecessary.
The HTML Strikethrough Element (<strike>) renders text with a strikethrough, or a line through it. This element is obselete in HTML5. Use the <del> instead if the element is marking an edit (deleted text), otherwise use an <s> element.
The HTML template element <template> is a mechanism for holding client-side content that is not to be rendered when a page is loaded but may subsequently be instantiated during runtime using JavaScript.
The HTML Teletype Text Element (<tt>) produces an inline element displayed in the browser's default monotype font. This element was intended to style text as it would display on a fixed width display, such as a teletype. It probably is more common to display fixed width type using the <code> element.
The HTML Example Element (<xmp>) renders text between the start and end tags without interpreting the HTML in between and using a monospaced font. The HTML2 specification recommended that it should be rendered wide enough to allow 80 characters per line.
The itemidglobal attribute is the unique, global identifier of an item. itemid attributes can only be specified on elements that have both itemscopeand itemtypeattributes. Also, the itemid can only be specified on elements with an itemscope attribute whose corresponding itemtype refers to or defines a vocabulary that supports global identifiers.