It is important to note that laws regarding what is illegal to watch on the internet can vary by country and jurisdiction. However, there are certain types of content that are generally considered illegal to watch or possess online, and they include child pornography, revenge porn, and copyrighted material.

Child pornography is a heinous crime, and it is illegal to produce, distribute, possess, or view it in almost all countries. The possession or viewing of child pornography is often punishable by imprisonment, and those convicted of these crimes may also be required to register as sex offenders. Child pornography is a form of sexual exploitation that involves the use of children in sexually explicit activities, and it is a violation of their human rights.
Revenge porn, also known as non-consensual pornography, is the distribution of sexually explicit images or videos without the consent of the person depicted. Revenge porn is a violation of privacy and can cause significant harm to the victims, including emotional distress and reputational damage. Many countries have laws that prohibit the distribution of revenge porn, and those found guilty of these crimes may face imprisonment and fines.
Copyright infringement is the unauthorized use of copyrighted material, such as movies, music, and software. It is illegal to download or stream copyrighted material without permission from the copyright holder. While copyright laws vary by country, most have provisions that allow for the prosecution of individuals who engage in copyright infringement. The penalties for copyright infringement can include fines, imprisonment, and even seizure of property.
In addition to these specific types of content, there are also websites and online activities that are illegal. For example, websites that promote hate speech, racism, or terrorism may be illegal in some countries. Online scams and fraud are also illegal and can lead to significant financial loss for victims.
It is important to note that the legality of watching certain types of content may depend on the context in which it is viewed. For example, in some countries, it may be illegal to view certain types of content if it is done with the intention of sexually exploiting children, while in other countries, viewing this content may be legal if it is done for research or law enforcement purposes.
It is also worth noting that while it may not be illegal to watch certain types of content, it may still be morally reprehensible or harmful to others. For example, watching violent or disturbing videos may desensitize individuals to violence and contribute to a culture of violence.
It is the responsibility of individuals to be aware of the laws and regulations regarding online content in their country or jurisdiction. While the internet provides access to a wealth of information and entertainment, it is important to use it responsibly and with respect for the rights and well-being of others.