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 attribute

    Deprecated method of allowing scoped CSS styles using a "scoped" attribute. Now [removed from the specification](https://github.com/whatwg/html/issues/552) and replaced by the [@scope CSS rule](/css-cascade-scope).