Design
Responsive design
Responsive design is building a page so that its layout adapts to the width of the screen showing it, rather than shipping separate desktop and mobile sites. One page, every width.
The term dates from 2010 and the practice is now the default, but "responsive" and "works on a phone" are not the same thing. A layout that technically reflows can still push the important content below three screens of hero image, hide the navigation behind a menu nobody opens, or shrink a table into an unreadable strip.
The decisions that matter are about order and priority, not just width. On a narrow screen the columns stack, and something has to come first. If that order was never decided, the browser decides it by source order, which usually means whatever the desktop layout happened to put on the left.
The failure I see most often is a design signed off on a desktop mockup and then "made responsive" by whoever built it. Responsive behaviour is a design decision. It should be in the file, and where it is not, the build has to make it explicitly rather than by default.
Responsive intent in the handoffDesign