--- a/dom/base/DOMMatrix.cpp
+++ b/dom/base/DOMMatrix.cpp
@@ -37,36 +37,42 @@ JSObject* DOMMatrixReadOnly::WrapObject(
                                         JS::Handle<JSObject*> aGivenProto) {
   return DOMMatrixReadOnly_Binding::Wrap(aCx, this, aGivenProto);
 }
 
 already_AddRefed<DOMMatrixReadOnly> DOMMatrixReadOnly::Constructor(
     const GlobalObject& aGlobal,
     const Optional<StringOrUnrestrictedDoubleSequence>& aArg,
     ErrorResult& aRv) {
-  RefPtr<DOMMatrixReadOnly> rval =
-      new DOMMatrixReadOnly(aGlobal.GetAsSupports());
   if (!aArg.WasPassed()) {
+    RefPtr<DOMMatrixReadOnly> rval =
+        new DOMMatrixReadOnly(aGlobal.GetAsSupports());
     return rval.forget();
   }
 
   const auto& arg = aArg.Value();
   if (arg.IsString()) {
     nsCOMPtr<nsPIDOMWindowInner> win =
         do_QueryInterface(aGlobal.GetAsSupports());
     if (!win) {
       aRv.ThrowTypeError<MSG_ILLEGAL_CONSTRUCTOR>();
       return nullptr;
     }
+    RefPtr<DOMMatrixReadOnly> rval =
+        new DOMMatrixReadOnly(aGlobal.GetAsSupports());
     rval->SetMatrixValue(arg.GetAsString(), aRv);
-  } else {
-    const auto& sequence = arg.GetAsUnrestrictedDoubleSequence();
-    SetDataInMatrix(rval, sequence.Elements(), sequence.Length(), aRv);
+    return rval.forget();
   }
 
+  const auto& sequence = arg.GetAsUnrestrictedDoubleSequence();
+  const int length = sequence.Length();
+  const bool is2D = length == 6;
+  RefPtr<DOMMatrixReadOnly> rval =
+      new DOMMatrixReadOnly(aGlobal.GetAsSupports(), is2D);
+  SetDataInMatrix(rval, sequence.Elements(), length, aRv);
   return rval.forget();
 }
 
 already_AddRefed<DOMMatrixReadOnly> DOMMatrixReadOnly::ReadStructuredClone(
     nsISupports* aParent, JSStructuredCloneReader* aReader) {
   uint8_t is2D;
 
   if (!JS_ReadBytes(aReader, &is2D, 1)) {
@@ -504,39 +510,46 @@ static void SetDataInMatrix(DOMMatrixRea
     lengthStr.AppendInt(aLength);
     aRv.ThrowTypeError<MSG_MATRIX_INIT_LENGTH_WRONG>(lengthStr);
   }
 }
 
 already_AddRefed<DOMMatrix> DOMMatrix::Constructor(const GlobalObject& aGlobal,
                                                    const Float32Array& aArray32,
                                                    ErrorResult& aRv) {
-  RefPtr<DOMMatrix> obj = new DOMMatrix(aGlobal.GetAsSupports());
   aArray32.ComputeLengthAndData();
-  SetDataInMatrix(obj, aArray32.Data(), aArray32.Length(), aRv);
+
+  const int length = aArray32.Length();
+  const bool is2D = length == 6;
+  RefPtr<DOMMatrix> obj = new DOMMatrix(aGlobal.GetAsSupports(), is2D);
+  SetDataInMatrix(obj, aArray32.Data(), length, aRv);
 
   return obj.forget();
 }
 
 already_AddRefed<DOMMatrix> DOMMatrix::Constructor(const GlobalObject& aGlobal,
                                                    const Float64Array& aArray64,
                                                    ErrorResult& aRv) {
-  RefPtr<DOMMatrix> obj = new DOMMatrix(aGlobal.GetAsSupports());
   aArray64.ComputeLengthAndData();
-  SetDataInMatrix(obj, aArray64.Data(), aArray64.Length(), aRv);
+
+  const int length = aArray64.Length();
+  const bool is2D = length == 6;
+  RefPtr<DOMMatrix> obj = new DOMMatrix(aGlobal.GetAsSupports(), is2D);
+  SetDataInMatrix(obj, aArray64.Data(), length, aRv);
 
   return obj.forget();
 }
 
 already_AddRefed<DOMMatrix> DOMMatrix::Constructor(
     const GlobalObject& aGlobal, const Sequence<double>& aNumberSequence,
     ErrorResult& aRv) {
-  RefPtr<DOMMatrix> obj = new DOMMatrix(aGlobal.GetAsSupports());
-  SetDataInMatrix(obj, aNumberSequence.Elements(), aNumberSequence.Length(),
-                  aRv);
+  const int length = aNumberSequence.Length();
+  const bool is2D = length == 6;
+  RefPtr<DOMMatrix> obj = new DOMMatrix(aGlobal.GetAsSupports(), is2D);
+  SetDataInMatrix(obj, aNumberSequence.Elements(), length, aRv);
 
   return obj.forget();
 }
 
 already_AddRefed<DOMMatrix> DOMMatrix::ReadStructuredClone(
     nsISupports* aParent, JSStructuredCloneReader* aReader) {
   uint8_t is2D;
 
@@ -765,17 +778,19 @@ DOMMatrixReadOnly* DOMMatrixReadOnly::Se
   if (!ServoCSSParser::ParseTransformIntoMatrix(
           aTransformList, contains3dTransform, transform)) {
     aRv.Throw(NS_ERROR_DOM_SYNTAX_ERR);
     return nullptr;
   }
 
   if (!contains3dTransform) {
     mMatrix3D = nullptr;
-    mMatrix2D = new gfx::MatrixDouble();
+    if (!mMatrix2D) {
+      mMatrix2D = new gfx::MatrixDouble();
+    }
 
     SetA(transform._11);
     SetB(transform._12);
     SetC(transform._21);
     SetD(transform._22);
     SetE(transform._41);
     SetF(transform._42);
   } else {

--- a/dom/tests/mochitest/general/test_DOMMatrix.html
+++ b/dom/tests/mochitest/general/test_DOMMatrix.html
@@ -457,17 +457,17 @@ function testSkewYInPlace()
 
 function testCreateMatrix3D()
 {
   var m = new DOMMatrix([1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1]);
 
   // Should be initialised to identity
   cmpMatrix(m, [1, 0, 0, 1, 0, 0],
             "DOMMatrix should produce identity matrix");
-  is(m.is2D, true, "should not produce 3d matrix");
+  is(m.is2D, false, "should produce 3d matrix");
 
   m = new DOMMatrix([1, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0]);
 
   // Should be initialised to identity
   cmpMatrix(m, [1, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0],
             "DOMMatrix should produce identity matrix");
   is(m.is2D, false, "should produce 3d matrix");
 }
@@ -702,26 +702,26 @@ function testParsing()
   ok(CompareDOMMatrix(m2, m), "string parsing didn't match");
 
   m = new DOMMatrix();
   m.setMatrixValue("translate(10px, 20px) scale(.5, 2) rotate(45deg)");
   ok(CompareDOMMatrix(m2, m), "string parsing didn't match");
 }
 
 
-function testStringify() {
-    var m = new DOMMatrix();
-    var s = "" + m;
-    ok(s == "matrix" + formatMatrix(m), "stringifier 1 produced wrong result: " + s);
-    m.a = 100;
-    s = "" + m;
-    ok(s == "matrix" + formatMatrix(m), "stringifier 2 produced wrong result: " + s);
-    m.m43 = 200;
-    s = "" + m;
-    ok(s == "matrix3d" + formatMatrix(m), "stringifier 3 produced wrong result:" + s);
+function testStringify() {
+    var m = new DOMMatrix();
+    var s = "" + m;
+    ok(s == "matrix" + formatMatrix(m), "stringifier 1 produced wrong result: " + s);
+    m.a = 100;
+    s = "" + m;
+    ok(s == "matrix" + formatMatrix(m), "stringifier 2 produced wrong result: " + s);
+    m.m43 = 200;
+    s = "" + m;
+    ok(s == "matrix3d" + formatMatrix(m), "stringifier 3 produced wrong result:" + s);
 }
 
 window.addEventListener("load", main);
 
 </script>
 </pre>
 </body>
 </html>

--- a/testing/web-platform/meta/css/geometry/DOMMatrix-001.html.ini
+++ b/testing/web-platform/meta/css/geometry/DOMMatrix-001.html.ini
@@ -1,79 +1,10 @@
 [DOMMatrix-001.html]
-  [testConstructor0]
-    expected: FAIL
-
-  [testConstructor1]
-    expected: FAIL
-
-  [testConstructor2]
-    expected: FAIL
-
-  [testConstructor3]
-    expected: FAIL
-
-  [testConstructor4]
-    expected: FAIL
-
-  [testConstructor5]
-    expected: FAIL
-
-  [testConstructor6]
-    expected: FAIL
-
-  [testConstructor7]
-    expected: FAIL
-
-  [testConstructor8]
-    expected: FAIL
-
-  [testConstructor9]
-    expected: FAIL
-
-  [testConstructor10]
-    expected: FAIL
-
-  [testConstructor12]
-    expected: FAIL
-
-  [testConstructor13]
-    expected: FAIL
-
   [new DOMMatrix(undefined)]
     expected: FAIL
 
-  [new DOMMatrix(float32Array) 16 elements]
-    expected: FAIL
-
-  [new DOMMatrix(float64Array) 16 elements]
-    expected: FAIL
-
-  [new DOMMatrix(sequence) 16 elements]
-    expected: FAIL
-
-  [new DOMMatrix(sequence)]
-    expected: FAIL
-
-  [new DOMMatrix(matrix)]
-    expected: FAIL
-
-  [new DOMMatrixReadOnly(float32Array) 16 elements]
-    expected: FAIL
-
-  [new DOMMatrixReadOnly(float64Array) 16 elements]
-    expected: FAIL
-
-  [new DOMMatrixReadOnly(sequence) 16 elements]
-    expected: FAIL
-
-  [new DOMMatrixReadOnly(sequence)]
-    expected: FAIL
-
-  [new DOMMatrixReadOnly(matrix)]
-    expected: FAIL
-
   [new DOMMatrix("scale(2) translateX(5px) translateY(5px) rotate(5deg) rotate(-5deg)")]
     expected: FAIL
 
   [new DOMMatrixReadOnly("scale(2) translateX(5px) translateY(5px) rotate(5deg) rotate(-5deg)")]
     expected: FAIL