Q18: What is the ruling regarding visiting internet cafés, even though they have become – in the place where the individual resides – a gathering place for those who seek to commit sins over the internet such as appointments for prohibited meetings and the like? What is the ruling regarding working in it? What is the ruling regarding establishing a business project like this?
The above necessitates that visiting the mentioned café amounts to participation in the forbidden or encouraging it, so it is then prohibited, and earning any money this way is prohibited too.
Q19: What are the rulings regarding advertisements placed on the individual’s personal websites, whether he accepted them in return for certain financial payments or because the host of the webpage imposed it? Is it mandatory to refuse them, especially when some of them advertise prohibited things like wine, gambling and music or use lewd pictures and the like?
Allowing such advertisements is not prohibited by itself, whatever kind it may be, unless it is prohibited because of another reason like encouraging or promoting falsehood. This is similar to some of the previous answers. If it is prohibited then collecting money for it is prohibited too.
Q20: Do some of the internet webpages become subjected to the ruling of books of misguidance – which are prohibited to keep and read – with respect to the individual if he fears its negative effect on his beliefs or behavior and that of his children?
Yes, they have the same ruling as the books of misguidance, as what is prohibited is falling in misguidance through any means, books or otherwise.