1. seamless attribute for iframes

    The seamless attribute makes an iframe's contents actually part of a page, and adopts the styles from its hosting page. The attribute has been removed from both [the WHATWG](https://github.com/whatwg/html/issues/331) and [the W3C](https://github.com/w3c/html/pull/325) HTML5 specifications.

  2. Context menu item (menuitem element)

    Method of defining a context menu item, now deprecated and [removed from the HTML specification](https://github.com/whatwg/html/issues/2730).

  3. Scoped CSS

    Allows CSS rules to be scoped to part of the document, based on the position of the style element. The attribute has been [removed from the current specification](https://github.com/whatwg/html/issues/552).