We have two existing issues

• Implement Shadow DOM slots #23590
• Implement Shadow DOM event retargeting #23591

that are referring to an old spec that doesn't exist anymore. So I will close those so we can refocus here.

The shadow DOM seems to be specified in various places of the DOM spec, such as:

• https://dom.spec.whatwg.org/#dom-element-attachshadow
• https://dom.spec.whatwg.org/#interface-shadowroot

First question: what do we need to support the shadow dom?

There has been some recent work such as #34306 and #34298, and it would be good to have a list of TODO's in one issue for work that remains to be done.

For example, it appears to me that to support the shadow DOM, one needs to implement related concepts such as HTMLSlotElement and shadow-tree-slots. Since this is a meta issue, this could be the place to list all the various elements(pun intended) needed to implement shadow DOM, and supplement this with standalone issues for individual elements.

There are also other concepts that rely on the shadow DOM, such as Web Components, and we could list them here as well(if we plan on implementing them).