TPE DOM/Shadow DOM
"Shadow DOM provides encapsulation for DOM and CSS in a Web Component. Shadow DOM makes it so these things remain separate from the DOM of the main document. You can also use Shadow DOM by itself, outside of a web component." quotes from MDN.
- DOM part: Olli Pettay, Ben Tian, Jessica Jong
- Layout part: Emilio Cobos Alvarez
FF 60 might be a realistic target to ship, but there will be a intent-to-ship before that.