For Web Devs,
Good programming skills, and understanding on common web development technologies.
Good understanding on how things works at least the theoretical aspect, like how HTTP works, How a web application works, How to deploy a web application on a server, How REST APIS works etc.
These days, knowledge on declarative programming framework is also a must, at least how to work with frameworks like React.
Some DB knowledge, SQL knowledge
These are common for any internship
data structures and algorithm knowledge
problem solving skills
best practices, OOP concepts, knowledge on basic design patterns
how to write a clean and maintainable code
We don't look for specific technology or framework knowledge from an Intern. we expect them to learn the technology and complete their project. so we just evaluate the potential when interviewing interns.
(This might be vary from company to company)