The HTML Center Element (<center>) is a block-level element that can contain paragraphs and other block-level and inline elements. The entire content of this element is centered horizontally within its containing element (typically, the <body>).
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.
<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.