Fix source insertion/moving/removing steps

As discovered in #10657, no browser's removing or moving steps rely on the 'next sibling' pointer associated with the old element's position. After writing a test, we also discovered that the same goes for the insertion steps. In all cases, when the parent (old parent, for removing/moving steps) is a <picture> element, a relevant mutation is triggered for all <img> children of the parent. Update the spec to reflect this.

Closes #11113.

Author: domfarolino

source

@@ -29179,34 +29179,42 @@ interface <dfn interface>HTMLSourceElement</dfn> : <span>HTMLElement</span> {
   <var>insertedNode</var>, are:</p>
 
   <ol>
-   <li><p>If <var>insertedNode</var>'s parent is a <span>media element</span> that has no <code
+   <li><p>Let <var>parent</var> be <var>insertedNode</var>'s <span>parent</span>.</p></li>
+
+   <li><p>If <var>parent</var> is a <span>media element</span> that has no <code
    data-x="attr-media-src">src</code> attribute and whose <code
    data-x="dom-media-networkState">networkState</code> has the value <code
    data-x="dom-media-NETWORK_EMPTY">NETWORK_EMPTY</code>, then invoke that <span>media
    element</span>'s <span data-x="concept-media-load-algorithm">resource selection
    algorithm</span>.</p></li>
 
-   <li><p>If <var>insertedNode</var>'s next sibling is an <code>img</code> element and its parent is
-   a <code>picture</code> element, then, count this as a <span data-x="relevant mutations">relevant
-   mutation</span> for the <code>img</code> element.</p></li>
+   <li><p>If <var>parent</var> is a <code>picture</code> element, then <span data-x="list
+   iterate">for each</span> <var>child</var> of <var>parent</var>'s <span
+   data-x="concept-tree-child">children</span>, if <var>child</var> is an <code>img</code> element,
+   then count this as a <span data-x="relevant mutations">relevant mutation</span> for
+   <var>child</var>.</p></li>
   </ol>
 
   <p>The <code>source</code> <span>HTML element moving steps</span>, given <var>movedNode</var> and
   <var>oldParent</var>, are:</p>
 
   <ol>
-   <li><p>If <var>movedNode</var>'s next sibling was an <code>img</code> element and
-   <var>oldParent</var> is a <code>picture</code> element, then, count this as a <span
-   data-x="relevant mutations">relevant mutation</span> for the <code>img</code> element.</p></li>
+   <li><p>If <var>oldParent</var> is a <code>picture</code> element, then <span data-x="list
+   iterate">for each</span> <var>child</var> of <var>oldParent</var>'s <span
+   data-x="concept-tree-child">children</span>, if <var>child</var> is an <code>img</code> element,
+   then count this as a <span data-x="relevant mutations">relevant mutation</span> for
+   <var>child</var>.</p></li>
   </ol>
 
   <p>The <code>source</code> <span>HTML element removing steps</span>, given <var>removedNode</var>
   and <var>oldParent</var>, are:</p>
 
   <ol>
-   <li><p>If <var>removedNode</var>'s next sibling was an <code>img</code> element and
-   <var>oldParent</var> is a <code>picture</code> element, then, count this as a <span
-   data-x="relevant mutations">relevant mutation</span> for the <code>img</code> element.</p></li>
+   <li><p>If <var>oldParent</var> is a <code>picture</code> element, then <span data-x="list
+   iterate">for each</span> <var>child</var> of <var>oldParent</var>'s <span
+   data-x="concept-tree-child">children</span>, if <var>child</var> is an <code>img</code> element,
+   then count this as a <span data-x="relevant mutations">relevant mutation</span> for
+   <var>child</var>.</p></li>
   </ol>
 
   <p>The IDL attributes <dfn attribute for="HTMLSourceElement"><code