1. He watched some videos at the CNN website.

2. He watched some videos in the CNN website.

3. He watched some videos on the CNN website.

Which preposition is correct?

The last one is the standard idiom.

Number 3 is the most common usage. Some prepositions have such vague or multiple meanings that the "correct" preposition is more a matter of custom than of any specific wrong or right.

The preposition "on" is the correct option among the given choices. When referring to watching videos or consuming content on a website, we typically use the preposition "on" before the name of the website. Therefore, the correct sentence is: "He watched some videos on the CNN website." To determine correct prepositions in similar situations, you can consider the preposition commonly used with the specific website or platform you are referring to.