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diff --git a/testing/web-platform/tests/css/css-contain/container-queries/at-container-style-serialization.html b/testing/web-platform/tests/css/css-contain/container-queries/at-container-style-serialization.html
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+++ b/testing/web-platform/tests/css/css-contain/container-queries/at-container-style-serialization.html
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+<!doctype html>
+<title>CSS Container Queries: style() conditionText serialization</title>
+<link rel="help" href="https://drafts.csswg.org/css-contain-3/#container-queries">
+<link rel="help" href="https://drafts.csswg.org/cssom/#serialize-a-css-rule">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="support/cq-testcommon.js"></script>
+<style id="testSheet">
+  @container style(        --foo:bar) { }
+  @container STyle(--foo:    ) { }
+  @container STyle(--foo:) { }
+  @container STyle(--foo) { }
+  @container  style(  ( --FOO: BAR) OR ( prop: val  ) ) { }
+  @container style (--foo: bar) { }
+  @container style(--foo: bar   baz) { }
+  @container style(--foo:2.100  ) { }
+</style>
+<script>
+  setup(() => {
+    assert_implements_container_queries();
+    assert_equals(testSheet.sheet.cssRules.length, 8);
+  });
+
+  const tests = [
+      ["style(--foo: bar)", "Normalize spaces"],
+      ["style(--foo: )", "Empty declaration value - spaces"],
+      ["style(--foo: )", "Empty declaration value"],
+      ["style(--foo)", "No declaration value"],
+      ["style((--FOO: BAR) or ( prop: val ))", "Unknown CSS property after 'or'"],
+      ["style (--foo: bar)", "Not a style function with space before '('"],
+      ["style(--foo: bar   baz)", "Spaces preserved in custom property value"],
+      ["style(--foo: 2.100)", "Original string number in custom property value"]
+  ].map((e, i) => [testSheet.sheet.cssRules[i], ...e]);
+
+  tests.forEach((t) => {
+    test(() => assert_equals(t[0].conditionText, t[1]), t[2]);
+  });
+</script>