Why is it not recommended to place program code inside HTTP headers?

Embedding program code within HTTP headers is not recommended for several reasons:

• Security: HTTP headers are meant for transmitting metadata, not executable code. Embedding code can lead to vulnerabilities, such as XSS attacks.
• Protocol standards: HTTP headers should only contain textual data in a specific format. Code violates the structure and can cause processing errors.
• Caching and proxies: Headers are often cached and processed by intermediate nodes, which can lead to unexpected execution or blocking of code.
• Lack of support: Browsers and servers are not designed to execute code from headers, so such code simply will not work.

Therefore, program code should be placed in the body of the HTTP request or response, for example, in HTML or JavaScript files.