Responsive design stopped being a feature some years ago; it is now the baseline a product is judged against. What has changed is the range it has to cover — a single interface may be read on a phone held in one hand, on a laptop, and on a wall-mounted dashboard nobody ever touches.
One layout, many widths
We design the layout as a set of relationships rather than a set of screen sizes. Columns collapse in a defined order, type scales on a curve rather than in steps, and nothing depends on a breakpoint that a future device will land between.
Test on the hardware, not the simulator
Emulated viewports catch layout bugs. They do not catch a tap target that is comfortable with a mouse and impossible with a thumb, or a fixed header that eats half a short screen. Real devices catch both.
