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.
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).
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).
View Transitions API (single-document)
Provides a mechanism for easily creating animated transitions between different DOM states, while also updating the DOM contents in a single step. This API is specific to single-document transitions, support for same-origin cross-document transitions is [being planned](https://github.com/WICG/view-transitions/blob/main/cross-doc-explainer.md).