diff --git a/css/css-shadow-parts/simple-important-important.html b/css/css-shadow-parts/simple-important-important.html
new file mode 100644
index 00000000000000..43ec1bbae346e0
--- /dev/null
+++ b/css/css-shadow-parts/simple-important-important.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <title>CSS Shadow Parts - Simple Important Important</title>
+    <meta href="mailto:fergal@chromium.org" rel="author" title="Fergal Daly">
+    <link href="http://www.google.com/" rel="author" title="Google">
+    <link href="https://drafts.csswg.org/css-shadow-parts/" rel="help">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <script src="support/shadow-helper.js"></script>
+  </head>
+  <body>
+    <style>#c-e::part(partp) { color: green !important; }</style>
+    <script>installCustomElement("custom-element", "custom-element-template");</script>
+    <template id="custom-element-template">
+      <style>span { color: red !important; }</style>
+      <span id="part" part="partp">This text</span>
+    </template>
+    The following text should be green:
+    <custom-element id="c-e"></custom-element>
+    <script>
+      "use strict";
+      const colorRed = "rgb(255, 0, 0)";
+      test(function() {
+        const el = getElementByShadowIds(document, ["c-e", "part"]);
+        assert_equals(window.getComputedStyle(el).color, colorRed);
+      }, "Part in selected host is styled");
+    </script>
+  </body>
+</html>
diff --git a/css/css-shadow-parts/simple-important-inline.html b/css/css-shadow-parts/simple-important-inline.html
new file mode 100644
index 00000000000000..61b83dd4be7f4f
--- /dev/null
+++ b/css/css-shadow-parts/simple-important-inline.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <title>CSS Shadow Parts - Simple Important Inline</title>
+    <meta href="mailto:fergal@chromium.org" rel="author" title="Fergal Daly">
+    <link href="http://www.google.com/" rel="author" title="Google">
+    <link href="https://drafts.csswg.org/css-shadow-parts/" rel="help">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <script src="support/shadow-helper.js"></script>
+  </head>
+  <body>
+    <style>#c-e::part(partp) { color: red; }</style>
+    <script>installCustomElement("custom-element", "custom-element-template");</script>
+    <template id="custom-element-template">
+      <span id="part" part="partp" style="color: green !important">This text</span>
+    </template>
+    The following text should be red:
+    <custom-element id="c-e"></custom-element>
+    <script>
+      "use strict";
+      const colorGreen = "rgb(0, 128, 0)";
+      test(function() {
+        const el = getElementByShadowIds(document, ["c-e", "part"]);
+        assert_equals(window.getComputedStyle(el).color, colorGreen);
+      }, "Part in selected host is not styled");
+    </script>
+  </body>
+</html>
diff --git a/css/css-shadow-parts/simple-important.html b/css/css-shadow-parts/simple-important.html
new file mode 100644
index 00000000000000..e3240bd3905f86
--- /dev/null
+++ b/css/css-shadow-parts/simple-important.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <title>CSS Shadow Parts - Simple Important</title>
+    <meta href="mailto:fergal@chromium.org" rel="author" title="Fergal Daly">
+    <link href="http://www.google.com/" rel="author" title="Google">
+    <link href="https://drafts.csswg.org/css-shadow-parts/" rel="help">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <script src="support/shadow-helper.js"></script>
+  </head>
+  <body>
+    <style>#c-e::part(partp) { color: red; }</style>
+    <script>installCustomElement("custom-element", "custom-element-template");</script>
+    <template id="custom-element-template">
+      <style>span { color: green !important; }</style>
+      <span id="part" part="partp">This text</span>
+    </template>
+    The following text should be green:
+    <custom-element id="c-e"></custom-element>
+    <script>
+      "use strict";
+      const colorGreen = "rgb(0, 128, 0)";
+      test(function() {
+        const el = getElementByShadowIds(document, ["c-e", "part"]);
+        assert_equals(window.getComputedStyle(el).color, colorGreen);
+      }, "Part in selected host is styled");
+    </script>
+  </body>
+</html>
