Don't drop CSSStyleSheet.cssRules from CSSStyleSheet.replace/Sync().
As per spec this object should always return the same JS object.
Differential Revision: https://phabricator.services.mozilla.com/D144569
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1752392
gecko-commit: 842cc90cd23581a0785d9efd09889b95630ec781
gecko-reviewers: firefox-style-system-reviewers, layout-reviewers, boris
diff --git a/css/cssom/CSSStyleSheet-constructable-cssRules.html b/css/cssom/CSSStyleSheet-constructable-cssRules.html
new file mode 100644
index 0000000..72b0e15
--- /dev/null
+++ b/css/cssom/CSSStyleSheet-constructable-cssRules.html
@@ -0,0 +1,22 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSStyleSheet.replace/replaceSync() doesn't change cssRules object</title>
+<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
+<link rel="author" href="https://mozilla.org" title="Mozilla">
+<link rel="help" href="https://drafts.csswg.org/cssom/#extensions-to-the-document-or-shadow-root-interface">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1752392">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<span>Should be green</span>
+<script>
+promise_test(async function() {
+ const sheet = new CSSStyleSheet();
+ let rules = sheet.cssRules;
+ sheet.replaceSync('span {color:blue;}');
+ assert_equals(rules, sheet.cssRules, "Rules should be the same after replaceSync");
+ await sheet.replace('span {color: lime;}');
+ assert_equals(rules, sheet.cssRules, "Rules should be the same after replace()");
+ document.adoptedStyleSheets = [sheet];
+ assert_equals(getComputedStyle(document.querySelector("span")).color, "rgb(0, 255, 0)", "Sheet should apply");
+}, "cssRules doesn't change on replace / replaceSync");
+</script>
