<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.
<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 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 Element.closest() method returns the closest ancestor of the current element (or the current element itself) which matches the selectors given in parameter. If there isn't such an ancestor, it returns null.
The HTML Acronym Element (<acronym>) allows authors to clearly indicate a sequence of characters that compose an acronym or abbreviation for a word. This element has been removed in HTML5. Use <abbr> element.
The HTML <nobr> element prevents a text from breaking into a new line automatically, so it is displayed on one long line and scrolling might be necessary. This tag is not standard HTML and should not be used. Instead use the CSS property white-space like this:
The HTML Plaintext Element (<plaintext>) renders everything following the start tag as raw text, without interpreting any HTML. There is no closing tag, since everything after it is considered raw text.