Add test that getComputedStyle() works with !important

Closes #3532.

Author: asamuzaK

test/web-platform-tests/to-upstream/css/cssom/getComputedStyle-important.html

@@ -5,12 +5,19 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <!-- Regression test for https://github.com/jsdom/jsdom/issues/2485 -->
+<!-- Regression test for https://github.com/jsdom/jsdom/issues/3532 -->
 
 <style>
   #testElement { display: none !important; }
 </style>
-
+<style>
+  #testElement2 { display: none !important; }
+</style>
+<style>
+  #testElement2 { display: inline-block; }
+</style>
 <div id="testElement" style="display: block;"></div>
+<div id="testElement2"></div>
 
 <script>
 "use strict";
@@ -20,4 +27,10 @@
   const style = window.getComputedStyle(element);
   assert_equals(style.display, "none");
 }, "getComputedStyle should return !important stylesheet value over inline style");
+
+test(() => {
+  const element = document.getElementById("testElement2");
+  const style = window.getComputedStyle(element);
+  assert_equals(style.display, "none");
+}, "getComputedStyle should return !important stylesheet value rather than the subsequent stylesheet value");
 </script>