In 2018, the increasing prevalence of smartphones prompted a shift towards a mobile-first approach in web development, emphasizing the creation of websites optimized for mobile devices before adapting them for larger screens. This shift was driven by the fact that mobile internet usage had surpassed desktop usage, necessitating responsive design to ensure seamless user interface and experience across various devices. The text highlights challenges in achieving mobile compatibility, such as the need for responsive designs that adapt to different resolutions and the limitations of certain mobile browsers in supporting modern web technologies. It discusses different design strategies—fixed, adaptive, and fluid layouts—to enhance responsiveness and mentions frameworks like Preact and Bootstrap that aid in building mobile-friendly websites. Additionally, it notes unsupported features in mobile browsers, such as certain video formats and APIs, underscoring the importance of thorough testing and adaptation to achieve optimal mobile compatibility.