microwave

If you usually heat in an open container, then there is no difference between a situation where the liquid has spilled and the normal situation, since the microwave itself receives the heat vapor one way or another. The difference is only with the plate, and if it is made of glass, it does not absorb (but you need to check what material it is made of. I suppose there is room for leniency even with modern materials because they do not absorb in light of the findings of absorption experiments conducted in recent years). In principle, it is not customary to use the same microwave for dairy and meat, although some allow it. In any case, it is better in a closed container. If you do it in a closed container, then there is no problem in principle even without kosher. Beyond that, glass containers do not absorb water, but the body of the microwave seems to be made of metal, so it is better to kosher by heating an open container with water in it (after cleaning). But as mentioned, this is true even if the liquid does not flow out, but the container is open.