1 files changed, 70 insertions, 6 deletions
diff --git a/tests/wpt/web-platform-tests/css/cssom/CSSStyleSheet-constructable-duplicate.html b/tests/wpt/web-platform-tests/css/cssom/CSSStyleSheet-constructable-duplicate.html
index 30a3ed1d09b..579f5d0fced 100644
--- a/tests/wpt/web-platform-tests/css/cssom/CSSStyleSheet-constructable-duplicate.html
+++ b/tests/wpt/web-platform-tests/css/cssom/CSSStyleSheet-constructable-duplicate.html
@@ -4,17 +4,81 @@
 <link rel="help" href="https://wicg.github.io/construct-stylesheets/">
 <script src = '/resources/testharness.js'></script>
 <script src = '/resources/testharnessreport.js'></script>
-<div></div>
+<div id="target"></div>
 <script>
-test(function() {
-  let sheets = [];
 
-  for (let i = 0; i < 2; ++i) {
+function attachShadowDiv(host) {
+  const shadowRoot = host.attachShadow({mode: 'open'});
+  const shadowDiv = document.createElement("div");
+  shadowRoot.appendChild(shadowDiv);
+  return shadowDiv;
+}
+
+function blueSheetsWithIncreasingZIndex(n) {
+  let sheets = [];
+  for (let i = 0; i < n; ++i) {
     sheets.push(new CSSStyleSheet());
-    sheets[i].replaceSync("div { z-index: " + i + " }");
+    sheets[i].replaceSync("div { z-index: " + i + "; color: blue; }");
   }
+  return sheets;
+}
+
+let sheets = [];
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
 
   document.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1]];
   assert_equals(getComputedStyle(document.querySelector("div")).zIndex, "1", "duplicate stylesheet should take right position in the cascade");
-});
+
+  document.adoptedStyleSheets = [];
+}, "Duplicate stylesheets have the right cascade position in the Document");
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
+
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync("div { color: red; }");
+
+  document.adoptedStyleSheets = [sheets[1], sheets[0]];
+  assert_equals(getComputedStyle(document.querySelector("div")).zIndex, "0", "backmost stylesheet should take precedence");
+  assert_equals(getComputedStyle(document.querySelector("div")).color, "rgb(0, 0, 255)", "backmost stylesheet should take precedence");
+
+  document.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1], sheet];
+  assert_equals(getComputedStyle(document.querySelector("div")).zIndex, "1", "duplicate stylesheet should take the right position in the cascade");
+  assert_equals(getComputedStyle(document.querySelector("div")).color, "rgb(255, 0, 0)", "backmost stylesheet should take precedence");
+
+  document.adoptedStyleSheets = [];
+}, "Appending duplicate stylesheets yields the correct cascade position in the Document");
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
+
+  const span = document.createElement("span");
+  target.appendChild(span);
+  attachShadowDiv(span);
+
+  span.shadowRoot.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1]];
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).zIndex, "1", "duplicate stylesheet should take right position in the cascade");
+}, "Duplicate stylesheets have the right cascade position in the ShadowRoot");
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
+
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync("div { color: red; }");
+
+  const span = document.createElement("span");
+  target.appendChild(span);
+  attachShadowDiv(span);
+
+  span.shadowRoot.adoptedStyleSheets = [sheets[1], sheets[0]];
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).zIndex, "0", "backmost stylesheet should take precedence");
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).color, "rgb(0, 0, 255)", "backmost stylesheet should take precedence");
+
+  span.shadowRoot.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1], sheet];
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).zIndex, "1", "duplicate stylesheet should take right position in the cascade");
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).color, "rgb(255, 0, 0)", "backmost stylesheet should take precedence");
+}, "Appending duplicate stylesheets yields the correct cascade position in the ShadowRoot");
+
 </script>