Responsive Web Design is an architectural and technical approach to web design that ensures that the visuals and functionality of a web page automatically adapt and optimize to the size and characteristics of the user’s device screen. This method uses specialized HTML and CSS markup to provide a uniform user experience regardless of whether the user is using a desktop computer, tablet, or mobile phone.
Conceptually, responsive design is a fundamental principle of modern SEO. Google has officially declared the mobile version of the site as the main one for indexing and ranking (the so-called Mobile-First Indexing). This means that a good ranking of a site directly depends on its ability to offer an excellent experience on mobile devices.
Responsive design solves the problem of duplicate content (which would arise with a separate mobile version – https://www.google.com/search?q=m.domain.com) and improves key factors such as loading speed and usability. It is a mandatory requirement for any site that aims for high organic visibility in search engines.