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diff --git a/testing/web-platform/tests/css/geometry/DOMMatrix-stringifier.html b/testing/web-platform/tests/css/geometry/DOMMatrix-stringifier.html
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+++ b/testing/web-platform/tests/css/geometry/DOMMatrix-stringifier.html
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+<!DOCTYPE html>
+<title>Geometry Interfaces: DOMMatrix and DOMMatrixReadOnly stringifier</title>
+<link rel="help" href="https://drafts.fxtf.org/geometry/#DOMMatrix">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+["DOMMatrix", "DOMMatrixReadOnly", "WebKitCSSMatrix"].forEach(constr => {
+  const prefix = `${constr} stringifier:`;
+
+  // Basic
+  test(() => {
+    const matrix = new self[constr]();
+    assert_equals(String(matrix), "matrix(1, 0, 0, 1, 0, 0)");
+  }, `${prefix} identity (2d)`);
+
+  test(() => {
+    const matrix = self[constr].fromMatrix({is2D: false});
+    assert_equals(String(matrix), "matrix3d(1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1)");
+  }, `${prefix} identity (3d)`);
+
+  // Non-finites
+  [NaN, Infinity, -Infinity].forEach(num => {
+    test(() => {
+      const matrix = new self[constr]([1, 0, 0, 1, 0, num]);
+      assert_throws_dom("InvalidStateError", () => String(matrix));
+    }, `${prefix} ${num} (2d)`);
+
+    test(() => {
+      const matrix = new self[constr]([1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, num, 0, 1]);
+      assert_throws_dom("InvalidStateError", () => String(matrix));
+    }, `${prefix} ${num} (3d)`);
+  });
+
+  // Precision
+  [
+    ['1/3', '0.3333333333333333'],
+    ['1/300000', '0.0000033333333333333333'],
+    ['1/300000000', '3.3333333333333334e-9'],
+    ['100000 + (1/3)', '100000.33333333333'],
+    ['Math.pow(2, 53) + 1', '9007199254740992'],
+    ['Math.pow(2, 53) + 2', '9007199254740994'],
+    ['Number.MAX_VALUE', '1.7976931348623157e+308'],
+    ['Number.MIN_VALUE', '5e-324'],
+  ].forEach(([input, expected]) => {
+    const num = eval(input);
+    test(() => {
+      const matrix = new self[constr]([1, 0, 0, 1, 0, num]);
+      assert_equals(String(matrix), `matrix(1, 0, 0, 1, 0, ${expected})`);
+    }, `${prefix} ${input} (2d)`);
+
+    test(() => {
+      const matrix = new self[constr]([1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, num, 0, 1]);
+    assert_equals(String(matrix), `matrix3d(1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, ${expected}, 0, 1)`);
+    }, `${prefix} ${input} (3d)`);
+  });
+
+  // Should not invoke getters for m11 etc
+  function defineThrowingGetters(matrix) {
+    Object.defineProperty(matrix, 'a', {
+      get: () => assert_unreached('getter for a')
+    });
+    Object.defineProperty(matrix, 'm11', {
+      get: () => assert_unreached('getter for m11')
+    });
+  }
+
+  test(() => {
+    const matrix = new self[constr]();
+    defineThrowingGetters(matrix);
+    assert_equals(String(matrix), "matrix(1, 0, 0, 1, 0, 0)");
+  }, `${prefix} throwing getters (2d)`);
+
+  test(() => {
+    const matrix = self[constr].fromMatrix({is2D: false});
+    defineThrowingGetters(matrix);
+    assert_equals(String(matrix), "matrix3d(1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1)");
+  }, `${prefix} throwing getters (3d)`);
+});
+</script>