Mobile-First Breakpoint Cascade Simulator (2D)

This is the same real cascade-resolution model as the 3D version, rendered as a flat mock layout instead of a 3D tower. An ordered, user-editable list of min-width breakpoints — each overriding only some of columns / font-size / padding — is resolved exactly the way a browser resolves competing CSS rules: walk the list in ascending order, apply every rule whose threshold the current viewport satisfies, and let later rules win per property, not per breakpoint. Drag the viewport slider and the mock layout genuinely reflows to the freshly computed values, while the breakdown panel names the exact breakpoint that contributed each final property.