Cats in general are clean animals. They are very particular about grooming themselves and can spend many hours a day doing so. Hence, it is rather unfair to accuse them of dirtying the environment when it is actually inconsiderate fellow humans doing so. Some 'cat feeders' do not clean up after feeding the strays.
Then there is also accusation that the strays shit and urine all over. So far, I have never come across such incident. What I have witnessed is inconsiderate dogs' owners who do not clean up after their pets have done their 'business'. You can see them leaving urine marks at their favourite pillars at the void deck. If the owners are considerate, they should train their pets to discharge at grass patches or even into the drainage holes.
Just that day I saw a stray cat which discharged its urine into a drainage hole at the void deck. I wonder if it is an abandon cat who has been formally toilet trained. Even if it is so, I am impressed by its intelligent to be able to transfer what it has learned into a new environment.
Dogs are supposed to be more 'trainable' than cats. If a stray cat can maintain such high standard of hygiene and cleanliness, surely pets' owners could do better in up keeping the environment when walking their dogs.