Enforce mode matching for attachShadow() on declarative shadow root

This changes the behavior when calling `attachShadow()` on a node with an existing declarative shadow root. Now, if mode does not match between the arguments to `attachShadow()` and the existing root, throw an exception. Prior behavior was to silently return the declarative root as-is, with mismatched parameters.

Additionally, remove the note as it was incorrect.

See also: whatwg/html#10069.

Fixes #1235.

Co-authored-by: Anne van Kesteren <annevk@annevk.nl>

dom.bs

@@ -4502,6 +4502,8 @@ dom-Range-extractContents, dom-Range-cloneContents -->
  <a for=ShadowRoot>clonable</a> is true:
 
  <ol>
+ <li><p>Assert: <var>copy</var> is not a <a for=Element>shadow host</a>.
+
  <li><p>Run <a>attach a shadow root</a> with <var>copy</var>, <var>node</var>'s
  <a for=Element>shadow root</a>'s <a for=ShadowRoot>mode</a>, true, <var>node</var>'s
  <a for=Element>shadow root</a>'s <a for=ShadowRoot>delegates focus</a>, and <var>node</var>'s
@@ -6929,7 +6931,7 @@ are:
 <div algorithm>
 <p>To <dfn id=concept-attach-a-shadow-root>attach a shadow root</dfn>, given an
 <a for=/>element</a> <var>element</var>, a string <var>mode</var>, a boolean <var>clonable</var>,
-a boolean <var>delegatesFocus</var>, and a boolean <var>slotAssignment</var>:
+a boolean <var>delegatesFocus</var>, and a string <var>slotAssignment</var>:
 
 <ol>
  <li><p>If <var>element</var>'s <a for=Element>namespace</a> is not the <a>HTML namespace</a>,
@@ -6958,24 +6960,30 @@ a boolean <var>delegatesFocus</var>, and a boolean <var>slotAssignment</var>:
  <p>If <var>element</var> is a <a for=Element>shadow host</a>, then:
 
  <ol>
- <li><p>If <var>element</var>'s <a for=Element>shadow root</a>'s <a for=ShadowRoot>declarative</a>
- is false, then <a>throw</a> an "{{NotSupportedError!!exception}}" {{DOMException}}.
+ <li><p>Let <var>currentShadowRoot</var> be <var>element</var>'s <a for=Element>shadow root</a>.
+
+ <li>
+ <p>If any of the following are true:
+
+ <ul>
+ <li><p><var>currentShadowRoot</var>'s <a for=ShadowRoot>declarative</a> is false; or
+
+ <li><p><var>currentShadowRoot</var>'s <a for=ShadowRoot>mode</a> is not <var>mode</var>,
+ </ul>
+
+ <p>then <a>throw</a> a "{{NotSupportedError!!exception}}" {{DOMException}}.
 
  <li>
  <p>Otherwise:
 
  <ol>
- <li><p><a for=/>Remove</a> all of <var>element</var>'s <a for=Element>shadow root</a>'s
- <a for=tree>children</a>, in <a>tree order</a>.
+ <li><p><a for=/>Remove</a> all of <var>currentShadowRoot</var>'s <a for=tree>children</a>, in
+ <a>tree order</a>.
 
- <li><p>Set <var>element</var>'s <a for=Element>shadow root</a>'s
- <a for=ShadowRoot>declarative</a> to false.
+ <li><p>Set <var>currentShadowRoot</var>'s <a for=ShadowRoot>declarative</a> to false.
 
  <li><p>Return.
  </ol>
-
- <p class=note>This means that if multiple declarative shadow roots are contained within a single
- shadow host, only the last one will remain.
  </ol>
 
  <li><p>Let <var>shadow</var> be a new <a for=/>shadow root</a> whose <a for=Node>node document</a>