Whirlpool MH6150XMS Microwave Oven "clean brown spots door" Questions

...brown spots on the inside of the glass door. I cannot clean off these burn spots on the outside of the door, nor the inside of the door. It actually seems that the spots are inside the glass. I'm ...

...brown spots on the inside of the glass door. I cannot clean off these burn spots on the outside of the door, nor the inside of the door. It actually seems that the spots are inside the glass. I'm ...