y that width/height attributes are not settable
    shouldThrow("placeholder.width = 50");
    shouldThrow("placeholder.height = 60");

    assertWidthAndHeight(placeholder, "placeholder canvas after size reset", 30, 40);

    createImageBitmap(placeholder).then(image => {
      // Verify that an image grabbed from the placeholder has the correct dimensions
      assertWidthAndHeight(image, "imagebitmap from placeholder canvas", 30, 40);
      asyncStepsCompleted++;
      if (asyncStepsCompleted == 2) {
   