What key data is transmitted from the server to the client?

JSON objects with data necessary for display on the interface, client-side logic execution, or state preservation.

Headers containing metadata about the request: content type, response status (200 OK, 404 Not Found, etc.), cookies, caching.

Static files: HTML pages, CSS styles, JavaScript scripts, images, fonts.

Download files: documents, archives, multimedia files.

Redirects: instructions for the browser to navigate to another URL.

Errors: information about issues that occurred on the server (error text, code).