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diff --git a/testing/web-platform/tests/css/css-fonts/font-stretch-interpolation-math-functions.html b/testing/web-platform/tests/css/css-fonts/font-stretch-interpolation-math-functions.html
new file mode 100644
index 0000000000..74c42587f5
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+++ b/testing/web-platform/tests/css/css-fonts/font-stretch-interpolation-math-functions.html
@@ -0,0 +1,47 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>font-size-adjust interpolation with css math functions</title>
+    <link rel="help" href="https://drafts.csswg.org/css-fonts-5/#font-size-adjust-prop">
+    <link rel="help" href="https://drafts.csswg.org/css-values/#sign-funcs">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <script src="/css/support/interpolation-testcommon.js"></script>
+  </head>
+
+  <style>
+    :root {
+      font-size: 10px;
+    }
+  </style>
+  <body>
+    <script>
+      test_interpolation({
+        property: 'font-stretch',
+        from: '100%',
+        to: 'calc(sign(20rem - 20px) * 180%)',
+      }, [
+        {at: -1, expect: '20%'},
+        {at: 0, expect: '100%'},
+        {at: 0.125, expect: '110%'},
+        {at: 0.875, expect: '170%'},
+        {at: 1, expect: '180%'},
+        {at: 2, expect: '260%'}
+      ]);
+
+      test_interpolation({
+        property: 'font-stretch',
+        from: 'calc(sign(20rem - 20px) * 100%)',
+        to: 'calc(sign(20rem - 20px) * 180%)',
+      }, [
+        {at: -1, expect: '20%'},
+        {at: 0, expect: '100%'},
+        {at: 0.125, expect: '110%'},
+        {at: 0.875, expect: '170%'},
+        {at: 1, expect: '180%'},
+        {at: 2, expect: '260%'}
+      ]);
+    </script>
+  </body>
+</html>