What is a responsive website?

A responsive website is built with flexible layouts, using CSS techniques like media queries and fluid grids, so that it automatically rearranges and resizes its content to fit any screen size, from a small phone to a large desktop monitor, from a single underlying codebase. Rather than building and maintaining separate versions of a site for mobile and desktop (an older, more labour-intensive approach), responsive design defines breakpoints, specific screen-width thresholds at which the layout reflows, for example collapsing a three-column desktop layout into a single stacked column on a phone screen.

This matters practically because it ensures every visitor gets an appropriately formatted experience regardless of device, without the business needing to build and update multiple separate sites, and it’s also what Google uses, since Google’s indexing and ranking systems evaluate the mobile version of a site as the primary version for ranking purposes (mobile-first indexing).

Responsive design has effectively become the industry default rather than an optional feature, since the cost of maintaining separate mobile and desktop sites, and the SEO and usability impact of a poor mobile experience, make a single responsive codebase the practical standard.