A responsive website design agency should deliver a website that works smoothly on all devices, including mobile, tablet, and desktop. They focus on creating a user-friendly layout that automatically adjusts to different screen sizes. You can expect modern UI/UX design, fast loading speed, and clean navigation for better user experience. A professional agency also ensures SEO-friendly structure so your website can rank better in search engines. They usually provide cross-browser compatibility and thorough testing before launch. Additionally, they offer ongoing support and updates to keep your website optimized and secure.