"Fully responsive" is broader than media queries — it means the app adapts every dimension to the device: layout, content, interaction, media, and the payload itself.

1. Responsive layout

• Mobile-first, fluid by default — flex/grid, %/fr/rem, clamp(), minmax(auto-fit).
• Container queries so components respond to their space, not the viewport — essential for true reusability across layouts.
• A small, content-driven set of breakpoints.
• <meta viewport>, box-sizing: border-box.

2. Responsive content

• Show/hide or reprioritize content by viewport — but don't hide critical content from mobile users; reorganize, don't amputate.
• Truncate, collapse, or progressively disclose dense content on small screens.
• Navigation adapts: full nav → hamburger/bottom-nav.

3. Responsive interaction

• Touch vs pointer — touch targets ≥ ~44px; never depend on hover (use @media (hover: hover)).
• Gestures (swipe) on touch, keyboard affordances on desktop.
• Respect prefers-reduced-motion, prefers-color-scheme, prefers-contrast.

4. Responsive media

• srcset/sizes and <picture> — don't ship a 2000px image to a 375px phone.
• aspect-ratio + dimensions to prevent layout shift.
• loading="lazy" for offscreen media; modern formats (AVIF/WebP).

5. Responsive performance — the part people forget

A "responsive" app that ships 3MB of JS to a low-end Android isn't responsive in any meaningful sense.

• Adaptive loading — lighter experiences on slow networks / low-end devices (navigator.connection, deviceMemory).
• Code-split, lazy-load, defer non-critical work.
• Test on real low-end devices and throttled networks, not just a resized desktop window.

6. Verify

• Real devices across the range, landscape/portrait, accessibility zoom (200%+), slow 3G, and assistive tech.

The principle

Responsiveness is device empathy: the experience should be excellent on a flagship phone and a three-year-old budget Android on a flaky connection. Layout is just the most visible 20% of it.