Which of the following methods would not help in preventing XSS attacks?

Storing login credentials in cookies for security does not directly relate to preventing Cross-Site Scripting (XSS) attacks. XSS is a security vulnerability that allows an attacker to inject malicious scripts into content that is then served to users. While securely storing credentials is crucial for overall web application security, it does not mitigate the risk of script injections that XSS entails.

On the other hand, methods such as sanitizing input data, validating user input, and escaping output data are all proactive steps specifically aimed at reducing the risk of XSS vulnerabilities. Sanitizing input removes or neutralizes dangerous characters from user input, while validating user input ensures that data adheres to expected formats and conditions, preventing harmful scripts from being processed. Escaping output data makes sure that any potentially malicious code is presented as plain text rather than executed, thereby effectively mitigating any XSS risks.

Choosing to store login credentials in cookies, while generally a good security practice, does not address the mechanisms through which XSS attacks can be performed, making it the least effective approach in this context.