West Chester Borough ordinances require waterproof lids on trashcans:
§ 62-3 Disposal regulations…F
Rubbish or refuse shall be stored and placed out for collection in waterproof metal or plastic containers with handles which shall not exceed a capacity of 35 gallons. All containers shall be provided with tight-fitting lids which are windproof, ratproof and flyproof. The lid shall not, however, be roped, chained or permanently affixed to the container unless hinged by the factory. All containers must contain the street address of the owner in clear, legible and conspicuous writing on the outside of the container. Any garbage, rubbish or refuse that is not contained and/or placed in a waterproof metal or plastic container with handles not exceeding a capacity of 35 gallons or in a disposal bag will not be collected by the Borough.
If lids are missing, water can accumulate in the open trash container unless it has holes drilled in the bottom. And furthermore, even if the can is turned over, an unattended lid can gather water and host mosquito larvae: