The significance of using a mobile-first design in web development is centered on enhancing user experience across mobile devices. As mobile internet usage continues to rise, prioritizing mobile design ensures that websites load quickly, are easy to navigate, and provide accessible content. A mobile-first approach often involves optimizing layouts, images, and functionality for smaller screens, improving usability and engagement. This design strategy not only meets user expectations but also impacts search engine ranking positively, as search engines prioritize mobile-friendly websites in their results.