Build UI that holds up on small screens, not just desktop. Use this skill whenever you are writing or reviewing HTML/CSS/React/Tailwind layout code, building any web page, component, landing page, dashboard, or form, OR when the user reports that something "breaks on mobile," "looks fine on desktop but not phone," has horizontal scroll, overflow, squished/overlapping elements, tiny tap targets, or text that doesn't fit. Trigger this even when the user doesn't say the word "responsive". Any time you generate front-end layout, apply these rules by default so the output works from 320px up.
