Re: [whatwg/dom] Introduce `moveBefore()` state-preserving atomic move API (PR #1307)

@domfarolino commented on this pull request.



> @@ -2652,6 +2652,50 @@ of a <var>node</var> into a <var>parent</var> before a <var>child</var>, run the
  <!-- Technically this is post-insert. -->
 </ol>
 
+<p>To <dfn export for=Node id=concept-node-ensure-pre-move-validity>ensure pre-move validity</dfn>
+of a <var>node</var> into a <var>parent</var> before a <var>child</var>, run these steps:
+
+<ol>
+ <li><p>If either <var>parent</var> or <var>node</var> are not <a>connected</a>, then
+ <a>throw</a> a "{{HierarchyRequestError!!exception}}" {{DOMException}}.</p></li>

I believe we discussed this at TPAC. The idea is that `moveBefore()` is used to preserve state, so in cases where state cannot be preserved, we should not just fall back to usual `insertBefore()`-style behavior. Instead, we'll throw and let the developer handle this case themselves. Basically we let `moveBefore()` succeed only when we're sure we can perform a state-preserving move, and when disconnected destinations or origins are involved, the state cannot be preserved.

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