A responsive website adapts its layout and interactions to the available space. Mobile-first is a way to build that behaviour: start with a usable narrow layout, then add enhancements as the viewport grows. It does not mean hiding essential information from desktop users or treating a phone as a scaled-down computer.

Start with the job, not the device

List what visitors need to accomplish: read an article, compare offers, submit a form or find contact details. On a narrow screen, order the content around that job. A decorative hero is secondary to a clear heading, useful context and the primary action.

Check the source order as well as the visual order. CSS can rearrange items visually while keyboard and assistive technology users encounter the document in a different sequence. Aim for an order that makes sense before applying layout changes.

Make the grid flexible

Give the main container a maximum width and flexible side margins.…