DOM Parts Proposal Additions (#1015)

* Split DOM Parts proposal for move into imperative document.

1. A new motivation section
2. New option for groups
3. New option for clone.
4. New option for partial attribute

* Move imperative parts of DOM-Parts.md to DOM-Parts-Imperative.md

Reorganize sections to put groups and cloning first.

* Introduce a declarative DOM parts proposal for <template>

* kenchris@ review.

Fix typo, rename PartialAttributePart to AttributePartFragment.

proposals/DOM-Parts-Declarative-Template.md

@@ -0,0 +1,73 @@
+# DOM Parts Declarative Template API
+
+This proposal covers the declarative API for creating a DOM part within a
+`<template>` element.
+
+## Proposal
+
+Double curly braces `{{}}` provide markers for DOM parts.
+
+In the most basic level, this proposal can produce the three DOM parts:
+`NodePart`, `AttributePart`, `ChildNodePart`.
+
+### Basic Examples
+
+Suppose we had the following template in some HTML extension template languages
+where `{name}` and `{email}` indicated locations of dynamic data insertion:
+
+```html
+<section>
+  <h1 id="name">{name}</h1>
+  Email: <a id="link" href="mailto:{email}">{email}</a>
+</section>
+```
+
+And the application has produce a template with the following content:
+
+```html
+<template>
+  <section>
+    <h1 id="name">{{}}</h1>
+    Email: <a id="link" href="{{}}">{{}}</a>
+  </section>
+</template>
+```
+
+This will create a `ChildNodePart` attached to `<h1>` with no content, a
+`NodePart` attached to `<a>`, and an `AttributePart` connected to `href`.
+
+A framework could fetch these parts use either a
+[`DocumentPart`](./DOM-Parts-Imperative.md#option-2-documentpartgroup) on the
+`DocumentFragment` produced by the template.
+
+## `PartialAttributePart`
+
+Allowing `{{}}` inside an attribute with some static content raises the same
+question as a
+[`PartialAttributePart`](./DOM-Parts-Imperative.md#partial-attribute-updates),
+but if the imperative questions are resolved so too could the declarative
+template syntax.
+
+## Metadata
+
+Templating systems may need to serialize data about the nodes they are marking
+into the processing instructions. Or at the very least parts could be named so
+that they are easier to fetch.
+
+```html
+<div>{{email data="foo"}}</div>
+```
+
+This could be exposed on the imperative API to be consumed in JavaScript by
+application logic.
+
+## Choice of marker
+
+The `{{}}` is a reasonable DOM part marker, but it could be something else, or
+it could even be something that is declared as the placeholder.
+
+## Compatability
+
+It may be challenging to implement `<template>` parsing of `{{}}` for
+compatability reasons, so there may need to be restrictions such as only
+allowing this template syntax in a new API like `createTemplateWithParts()`.