The dark grating on the door of a microwave is called a microwave shielding mesh. Its purpose is to prevent the microwaves from escaping the microwave oven and potentially causing harm to people or interfering with electronic devices nearby. The mesh is designed to block and contain the microwave radiation while allowing visible light to pass through, allowing you to see inside the oven.
The presence of a shielding mesh on the microwave door is a standard safety feature found in most microwave ovens, regardless of brand. It is an essential component to ensure that the microwaves are contained within the oven cavity during operation.