diff --git a/cjs/mixin/child-node.js b/cjs/mixin/child-node.js
index 364c23d0..4d6f8e51 100644
--- a/cjs/mixin/child-node.js
+++ b/cjs/mixin/child-node.js
@@ -16,23 +16,34 @@ const asFragment = (ownerDocument, nodes) => {
   return fragment;
 };
 
+// `node` itself can be one of `nodes`, so the insertion point is the nearest
+// sibling that is not moved (the spec's viable previous/next sibling).
 const before = (node, nodes) => {
   const {ownerDocument, parentNode} = node;
-  if (parentNode)
+  if (parentNode) {
+    let prev = node.previousSibling;
+    while (prev && nodes.includes(prev))
+      prev = prev.previousSibling;
+    const fragment = asFragment(ownerDocument, nodes);
     parentNode.insertBefore(
-      asFragment(ownerDocument, nodes),
-      node
+      fragment,
+      prev ? prev.nextSibling : parentNode.firstChild
     );
+  }
 };
 exports.before = before;
 
 const after = (node, nodes) => {
   const {ownerDocument, parentNode} = node;
-  if (parentNode)
+  if (parentNode) {
+    let next = getEnd(node)[NEXT];
+    while (next && nodes.includes(next))
+      next = getEnd(next)[NEXT];
     parentNode.insertBefore(
       asFragment(ownerDocument, nodes),
-      getEnd(node)[NEXT]
+      next
     );
+  }
 };
 exports.after = after;
 
diff --git a/esm/mixin/child-node.js b/esm/mixin/child-node.js
index ec8b55eb..a9a968d5 100644
--- a/esm/mixin/child-node.js
+++ b/esm/mixin/child-node.js
@@ -15,22 +15,33 @@ const asFragment = (ownerDocument, nodes) => {
   return fragment;
 };
 
+// `node` itself can be one of `nodes`, so the insertion point is the nearest
+// sibling that is not moved (the spec's viable previous/next sibling).
 export const before = (node, nodes) => {
   const {ownerDocument, parentNode} = node;
-  if (parentNode)
+  if (parentNode) {
+    let prev = node.previousSibling;
+    while (prev && nodes.includes(prev))
+      prev = prev.previousSibling;
+    const fragment = asFragment(ownerDocument, nodes);
     parentNode.insertBefore(
-      asFragment(ownerDocument, nodes),
-      node
+      fragment,
+      prev ? prev.nextSibling : parentNode.firstChild
     );
+  }
 };
 
 export const after = (node, nodes) => {
   const {ownerDocument, parentNode} = node;
-  if (parentNode)
+  if (parentNode) {
+    let next = getEnd(node)[NEXT];
+    while (next && nodes.includes(next))
+      next = getEnd(next)[NEXT];
     parentNode.insertBefore(
       asFragment(ownerDocument, nodes),
-      getEnd(node)[NEXT]
+      next
     );
+  }
 };
 
 export const replaceWith = (node, nodes) => {
diff --git a/test/html/element.js b/test/html/element.js
index 3c3fedd8..bf6ca456 100644
--- a/test/html/element.js
+++ b/test/html/element.js
@@ -35,6 +35,36 @@ node.replaceWith(amp);
 assert(amp.wholeText, 'before&after');
 assert(document.toString(), '<!DOCTYPE html><html lang="en" id="html" class="live">before&amp;after<input><p></p>ab</html>', '.before() and after()');
 
+{
+  const {document} = parseHTML('<div>a<i></i>b</div>');
+  const div = document.firstChild;
+  const i = div.querySelector('i');
+  i.after('x', i);
+  assert(div.outerHTML, '<div>ax<i></i>b</div>', '.after() with itself among the nodes');
+  i.before(i, 'y');
+  assert(div.outerHTML, '<div>ax<i></i>yb</div>', '.before() with itself among the nodes');
+  const b = div.lastChild;
+  i.after(b, 'z');
+  assert(div.outerHTML, '<div>ax<i></i>bzy</div>', '.after() moving the next sibling');
+  i.before(div.firstChild, i);
+  assert(div.outerHTML, '<div>xa<i></i>bzy</div>', '.before() moving the first child and itself');
+  i.before(i.previousSibling, 'p');
+  assert(div.outerHTML, '<div>xap<i></i>bzy</div>', '.before() moving the previous sibling');
+  i.after(i.nextSibling, 'n');
+  assert(div.outerHTML, '<div>xap<i></i>bnzy</div>', '.after() moving the next sibling first');
+}
+
+{
+  const {document} = parseHTML('<p>a<b></b>c</p>');
+  const p = document.firstChild;
+  const b = p.querySelector('b');
+  b.before(b.previousSibling);
+  b.after(b.nextSibling);
+  b.before(b, p.lastChild);
+  b.after(p.firstChild, b);
+  assert(p.outerHTML, '<p>a<b></b>c</p>', '.before() and .after() without a viable sibling');
+}
+
 let div = document.createElement('div');
 let rect = div.getBoundingClientRect();