diff --git a/css/cssom/CSSStyleSheet-constructable-concat-ref.html b/css/cssom/CSSStyleSheet-constructable-concat-ref.html
new file mode 100644
index 000000000000000..1c6b6784c9caa9a
--- /dev/null
+++ b/css/cssom/CSSStyleSheet-constructable-concat-ref.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Adoptedstylesheets.concat should work when starting empty</title>
+<link rel="author" href="mailto:masonf@chromium.org">
+
+<span>This should be green</span><br>
+<span>This should be green</span>
+<style>
+  span {
+    background-color:green;
+  }
+</style>
diff --git a/css/cssom/CSSStyleSheet-constructable-concat.html b/css/cssom/CSSStyleSheet-constructable-concat.html
new file mode 100644
index 000000000000000..3ba999284576143
--- /dev/null
+++ b/css/cssom/CSSStyleSheet-constructable-concat.html
@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Adoptedstylesheets.concat should work when starting empty</title>
+<link rel="author" href="mailto:masonf@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/cssom/#extensions-to-the-document-or-shadow-root-interface">
+<link rel="match" href="CSSStyleSheet-constructable-concat-ref.html">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<span>This should be green</span>
+<div id=host></div>
+
+<script>
+test(function() {
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync('span {background-color:green;}');
+  assert_equals(document.adoptedStyleSheets.length,0);
+  document.adoptedStyleSheets = document.adoptedStyleSheets.concat([sheet]);
+  assert_equals(document.adoptedStyleSheets.length,1);
+
+  const host = document.getElementById('host');
+  const shadow = host.attachShadow({mode: 'open'});
+  shadow.innerHTML = '<span>This should be green</span>';
+  assert_equals(shadow.adoptedStyleSheets.length,0);
+  shadow.adoptedStyleSheets = shadow.adoptedStyleSheets.concat([sheet]);
+  assert_equals(shadow.adoptedStyleSheets.length,1);
+}, "adoptedStyleSheets should allow .concat on empty starting values");
+</script>
diff --git a/css/cssom/adoptedstylesheets-observablearray.html b/css/cssom/adoptedstylesheets-observablearray.html
new file mode 100644
index 000000000000000..cb148c5ddfb00f3
--- /dev/null
+++ b/css/cssom/adoptedstylesheets-observablearray.html
@@ -0,0 +1,80 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Adoptedstylesheets as ObservableArray</title>
+<link rel="author" href="mailto:masonf@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/cssom/#extensions-to-the-document-or-shadow-root-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<span id=target>Test Span</span>
+
+<style>
+  #target {background-color: red;}
+</style>
+
+<script>
+function assert_is(targetStyle, color) {
+  assert_equals(targetStyle.getPropertyValue('background-color'), color);
+}
+
+function testRoot(d, targetStyle) {
+  const red = 'rgb(255, 0, 0)';
+  const green = 'rgb(0, 255, 0)';
+  const blue = 'rgb(0, 0, 255)';
+
+  const sheet1 = new CSSStyleSheet();
+  sheet1.replaceSync('#target {background-color:lime !important;}');
+  const sheet2 = new CSSStyleSheet();
+  sheet2.replaceSync('#target {background-color:blue !important;}');
+  assert_equals(d.adoptedStyleSheets.length, 0);
+  assert_is(targetStyle, red);
+
+  d.adoptedStyleSheets = [sheet1];
+  assert_equals(d.adoptedStyleSheets.length, 1);
+  assert_is(targetStyle, green);
+
+  d.adoptedStyleSheets.push(sheet2);
+  assert_equals(d.adoptedStyleSheets.length, 2);
+  assert_is(targetStyle, blue);
+
+  d.adoptedStyleSheets.pop();
+  assert_equals(d.adoptedStyleSheets.length, 1);
+  assert_is(targetStyle, green);
+
+  d.adoptedStyleSheets.push(sheet2);
+  d.adoptedStyleSheets.reverse();
+  assert_equals(d.adoptedStyleSheets.length, 2);
+  assert_is(targetStyle, green);
+
+  d.adoptedStyleSheets.splice(1, 1);
+  assert_equals(d.adoptedStyleSheets.length, 1);
+  assert_is(targetStyle, blue);
+  d.adoptedStyleSheets.splice(0, 1, sheet1);
+  assert_equals(d.adoptedStyleSheets.length, 1);
+  assert_is(targetStyle, green);
+}
+
+test(function() {
+  const target = document.querySelector('#target');
+  const targetStyle = window.getComputedStyle(target);
+  testRoot(document, targetStyle);
+}, "document.adoptedStyleSheets should allow mutation in-place");
+
+test(function() {
+  const host = document.createElement('div');
+  document.body.appendChild(host);
+  const shadow = host.attachShadow({mode: 'open'});
+  shadow.innerHTML = '<span id=target>Test Shadow Span</span><style>#target{background-color: red;}</style>';
+  const target = shadow.querySelector('#target');
+  const targetStyle = window.getComputedStyle(target);
+  testRoot(shadow, targetStyle);
+}, "shadowRoot.adoptedStyleSheets should allow mutation in-place");
+
+test(function() {
+  assert_true(Array.isArray(document.adoptedStyleSheets));
+  const host = document.createElement('div');
+  document.body.appendChild(host);
+  const shadow = host.attachShadow({mode: 'open'});
+  assert_true(Array.isArray(shadow.adoptedStyleSheets));
+}, "adoptedStyleSheets should return true for isArray()");
+</script>
