Broccoli being cooked in microwaveable dish in microwave in kitchen
Why Your Microwave Door Has A Mesh Grating
The mesh on your microwave door can make it hard to see the food inside, but it has a vital purpose: it keeps the radiation microwaves themselves from escaping through the glass.
The radiation can't pass through metal, which is also why the sides of your microwave are closed in with metal. These kinds of barriers are called Faraday cages.
The little holes in your microwave oven's mesh are actually smaller than microwave rays, keeping them in while letting visible light go out so you can partially see your food.
The mesh or polka-dot pattern on a conventional oven door has a different purpose. It stops the glass from shattering due to the heat, protecting you from injury.