Editorial: add sections on document and shadow trees

dom.bs

@@ -228,10 +228,10 @@ or null, and an ordered list of zero or more
 <a>parent</a> is object <var>B</var> is a
 <a>child</a> of <var>B</var>.
 
-The <dfn export for=tree id=concept-tree-root>root</dfn> of an object is itself, if its
-<a>parent</a> is null, or else it is the
-<a for=tree>root</a> of its
-<a>parent</a>.
+<p>The <dfn export for=tree id=concept-tree-root>root</dfn> of an object is itself, if its
+<a>parent</a> is null, or else it is the <a for=tree>root</a> of its <a>parent</a>. The
+<a for=tree>root</a> of a <a>tree</a> is any object <a for=tree>participating</a> in that
+<a>tree</a> whose <a for=tree>parent</a> is null.
 
 An object <var>A</var> is called a
 <dfn export for=tree id=concept-tree-descendant>descendant</dfn> of an object
@@ -1430,6 +1430,27 @@ A <a>node</a> is considered
 <a>length</a> is zero.
 
 
+<h4 id=document-trees>Document tree</h4>
+
+<p>A <dfn export id=concept-document-tree>document tree</dfn> is a <a>node tree</a> whose
+<a>root</a> is a <a>document</a>.
+
+<p>The <dfn export>document element</dfn> of a <a>document</a> is the <a for="/">element</a> whose
+<a>parent</a> is that <a>document</a>, if it exists, and null otherwise.
+
+<p class="note no-backref">Per the <a>node tree</a> constraints, there can be only one such
+<a for="/">element</a>.
+
+<p>An <a for=/>element</a> is <dfn export>in a document</dfn> if its <a>root</a> is a
+<a>document</a>.
+
+
+<h4 id=shadow-trees>Shadow tree</h4>
+
+<p>A <dfn export id=concept-shadow-tree>shadow tree</dfn> is a <a>node tree</a> whose <a>root</a> is
+a <a for=/>shadow root</a>.
+
+
 <h4 id=mutation-algorithms>Mutation algorithms</h4>
 
 To <dfn export for=Node id=concept-node-ensure-pre-insertion-validity>ensure pre-insertion validity</dfn>
@@ -4999,12 +5020,12 @@ interface ShadowRoot : DocumentFragment {
 <hr>
 
 <p>In <dfn export id=concept-shadow-including-tree-order>shadow-including tree order</dfn>, is
-<a>shadow-including preorder, depth-first traversal</a> of a <a>tree</a>.
-<dfn noexport>shadow-including preorder, depth-first traversal</dfn> of a <a>tree</a>
+<a>shadow-including preorder, depth-first traversal</a> of a <a>node tree</a>.
+<dfn noexport>shadow-including preorder, depth-first traversal</dfn> of a <a>node tree</a>
 <var>tree</var> is preorder, depth-first traversal of <var>tree</var>, with for each
-<a for=/>element</a> <var>element</var> encountered in <var>tree</var>, whose
-<a for=Element>shadow root</a> is non-null, <a>shadow-including preorder, depth-first traversal</a>
-of the <a for=/>element</a>'s <a for=Element>shadow root</a>'s <a>tree</a> just after it is
+<a for=/>element</a> <var>element</var> encountered in <var>tree</var> with a non-null
+<a for=Element>shadow root</a>, <a>shadow-including preorder, depth-first traversal</a> of that
+<a for=/>element</a>'s <a for=Element>shadow root</a>'s <a>node tree</a> just after it is
 encountered.
 
 <p>An object <var>A</var> is a
@@ -5361,18 +5382,6 @@ A <a>node</a>'s
 <a>parent</a> of a different type, its
 <a>parent element</a> is null.
 
-The <dfn export>document element</dfn> of a
-<a>document</a> is the
-<a for="/">element</a> whose
-<a>parent</a> is that
-<a>document</a>, if it exists, and null otherwise.
-
-<p class="note no-backref">Per the <a>node tree</a> constraints, there can only be one
-such <a for="/">element</a>.
-
-<p>An <a for=/>element</a> is <dfn export>in a document</dfn> if its <a>root</a> is a
-<a>document</a>.
-
 <hr>
 
 <dl class=domintro>