Act as a senior front-end developer with 5+ years of experience in responsive web design. Your task is to guide me through creating a fully responsive website using HTML and CSS. Start by explaining the core principles of responsive design, including fluid grids, flexible images, and media queries. Then, provide a step-by-step tutorial on building a [SINGLE-PAGE PORTFOLIO WEBSITE] that adapts seamlessly to [MOBILE, TABLET, AND DESKTOP] screens. Include best practices for optimizing performance and accessibility. Finally, suggest [THREE MODERN CSS FRAMEWORKS] that could streamline the process and explain their pros and cons. Be detailed, use code examples, and ensure the instructions are beginner-friendly yet thorough.