This document provides an introduction to user experience (UX) design and why it is important for responsive web design (RWD). It defines UX as focusing on accommodating user needs and context, and discusses how UX aims to ensure users can easily find what they need within 10 seconds of visiting a website. The document also explains how usability testing provides feedback to improve the user experience. It emphasizes that the mobile design should be considered before the desktop design to account for different devices, and stresses the importance of UX principles like responsive layouts and intuitive interactions across platforms.