Editorial: split attachShadow element list into separate definition (#1200)

* Split attachShadow element list into separate definition so it can be referenced from EditContext spec

* Stray newline

* Change to `valid shadow host name`

Co-authored-by: Anne van Kesteren <[email protected]>

* Update name in other location

* Export the new `<dfn>`.

Co-authored-by: Anne van Kesteren <[email protected]>

* nits

---------

Co-authored-by: Anne van Kesteren <[email protected]>

Author: dandclark

dom.bs

@@ -6809,39 +6809,38 @@ are:
  <a for=/>shadow root</a>'s <a for=ShadowRoot>mode</a> is "<code>open</code>", and null otherwise.
 </dl>
 
+<p>A <dfn export>valid shadow host name</dfn> is:
+
+<ul class=brief>
+ <li>a <a>valid custom element name</a>
+ <li>"<code>article</code>",
+ "<code>aside</code>",
+ "<code>blockquote</code>",
+ "<code>body</code>",
+ "<code>div</code>",
+ "<code>footer</code>",
+ "<code>h1</code>",
+ "<code>h2</code>",
+ "<code>h3</code>",
+ "<code>h4</code>",
+ "<code>h5</code>",
+ "<code>h6</code>",
+ "<code>header</code>",
+ "<code>main</code>",
+ "<code>nav</code>",
+ "<code>p</code>",
+ "<code>section</code>", or
+ "<code>span</code>"
+</ul>
+
 <p>The <dfn method for=Element><code>attachShadow(<var>init</var>)</code></dfn> method steps are:
 
 <ol>
  <li><p>If <a>this</a>'s <a for=Element>namespace</a> is not the <a>HTML namespace</a>,
  then <a>throw</a> a "{{NotSupportedError!!exception}}" {{DOMException}}.
 
- <li>
-  <p>If <a>this</a>'s <a for=Element>local name</a> is not one of the following:
-
-  <ul class=brief>
-   <li>a <a>valid custom element name</a>
-   <li>"<code>article</code>",
-   "<code>aside</code>",
-   "<code>blockquote</code>",
-   "<code>body</code>",
-   "<code>div</code>",
-   "<code>footer</code>",
-   "<code>h1</code>",
-   "<code>h2</code>",
-   "<code>h3</code>",
-   "<code>h4</code>",
-   "<code>h5</code>",
-   "<code>h6</code>",
-   "<code>header</code>",
-   "<code>main</code>",
-   "<code>nav</code>",
-   "<code>p</code>",
-   "<code>section</code>", or
-   "<code>span</code>"
-  </ul>
-
-  <p>then <a>throw</a> a "{{NotSupportedError!!exception}}" {{DOMException}}.
- </li>
+ <li><p>If <a>this</a>'s <a for=Element>local name</a> is not a <a>valid shadow host name</a>, then
+ <a>throw</a> a "{{NotSupportedError!!exception}}" {{DOMException}}.
 
  <li>
   <p>If <a>this</a>'s <a for=Element>local name</a> is a <a>valid custom element name</a>, or