Responsive websites are designed to adapt seamlessly across various devices and browsers, ensuring a consistent and user-friendly experience without layout disruptions or horizontal scrolling. The evolution of responsive web design began with Audi's 2001 layout that adjusted to browser viewports, progressing to the use of CSS media queries and the coining of the term in 2011. Effective responsive design relies on best practices such as prioritizing target devices, optimizing call-to-action elements, using scalable vector graphics (SVGs) for high-quality images, and ensuring clickable areas are appropriately sized for touchscreens. Lightweight frameworks like Skeleton and Foundation can be used instead of heavier options like Bootstrap to enhance loading times and SEO. A mobile-first approach is often recommended for easier scaling, and cross-browser testing is crucial to ensure compatibility across different devices. Tools like LambdaTest and LT Browser facilitate these tests by offering cloud-based testing across numerous browser-OS combinations. The blog also highlights exemplary responsive websites like those of Adobe, Netflix, and Slack, which demonstrate effective design and functionality across devices. Ultimately, understanding HTML and CSS fundamentals is essential for developing robust responsive websites, alongside conducting thorough responsiveness tests to ensure optimal performance.