Re: [w3c/webcomponents] Add an updated proposal for DOM Parts. (#899)

@annevk commented on this pull request.

I still need to read more, but this looks interesting and seems like a good iteration given the previous feedback of the approach being too high-level. Once thing that's not entirely clear to me is what the internal model of a part is and why having it is beneficial other than developer ergonomics. Does this enable any performance wins over mutating the tree yourself, for instance?

> +a *DOM part group* was represented by a [template element](https://html.spec.whatwg.org/multipage/scripting.html#the-template-element).
+This proposal keeps that approach possible but does not keep it the sole approach possible.
+
+### Interfaces
+
+```webidl
+interface Part {
+    attribute any value;
+    void commit();
+};
+
+interface NodePart : Part {
+    readonly attribute Node node;
+};
+
+interface AttributePart : NodePart {

It's a bit confusing to me that an attribute is a node (whereas the DOM mostly tries to pretend it's not) and a child node is not. Perhaps that's not what NodePart means, but you would think it is given how AttributePart and ChildNodePart are named.

> +and `AttributePart` for [`a` element](https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-a-element) as follows: 
+```js
+namePart = new ChildNodePart(name)
+emailPart = new ChildNodePart(link)
+emailAttributePart = new AttributePart(link, ’href’)
+
+Then assigning values as follows will update the DOM:
+namePart.value = "Ryosuke Niwa"
+emailPart.value = "rniwa@webkit.org"
+emailAttributePart.value = "mailto:rniwa@webkit.org"
+```
+
+The resultant DOM will look like this:
+```html
+<section><h1 id="name">Ryosuke Niwa</h1>Email: <a id="link" href="mailto:rniwa@webkit.org"></a></section>
+```

Why does this work without commit?

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