To avoid Cross-Site Scripting (XXS) vulnerabilities in web applications, it is crucial to implement a combination of input validation and output encoding practices. First, validate and sanitize all user inputs to ensure they conform to expected patterns and formats, using strict whitelisting and libraries designed for sanitization. Next, apply context-appropriate encoding before displaying user-generated content on web pages, such as HTML encoding with functions like htmlspecialchars() in PHP or html.escape() in Python, and JavaScript encoding with JSON.stringify(). Additionally, leverage security libraries and frameworks that offer built-in protection against XXS. Regularly update and patch your software, and conduct thorough security testing to identify and mitigate potential vulnerabilities. By adhering to these practices, you can significantly reduce the risk of XXS attacks on your web applications.