Responsive CSS: Mobile and Desktop at Once

Claude writes CSS for the desktop it's picturing and never checks the narrow viewport, so the same few things break every time: content overflows sideways, code blocks blow out the page, and text that's right on desktop is huge on a phone. Design for the small screen first and these mostly disappear.

Set the viewport, stop iOS inflating text

Without the viewport meta tag, mobile browsers render at a ~980px layout width and scale the result down, which is why everything looks oversized and mis-laid-out on a phone. This one line is non-negotiable on every page:

<meta name="viewport" content="width=device-width, initial-scale=1" />

And stop iOS from auto-enlarging text:

html { -webkit-text-size-adjust: 100%; text-size-adjust: 100%; }

The flex/grid min-width: 0 rule, this fixes most overflow

This is the single most common cause of mysterious horizontal scroll. Flex and grid children default to min-width: auto, which means they refuse to shrink below their content's intrinsic width. So one long line, a URL, or a <pre> inside a flex/grid item pushes the whole layout wider than the screen. Set min-width: 0 on the child (or overflow: hidden) and it shrinks correctly.

.flex-child, .grid-child { min-width: 0; }   /* lets long content shrink instead of overflowing */

If you fix nothing else, fix this. It's behind the code-block overflow in most "works on desktop, scrolls sideways on mobile" pages.

Code blocks scroll, they don't overflow the page

A <pre> doesn't wrap and has no scroll affordance by default, so long lines expand the page. Make the block scroll inside itself, and never line-wrap code (wrapping changes what the code means). The parent needs min-width: 0 per the rule above, or this still overflows.

pre {
  overflow-x: auto;     /* scroll inside the block */
  max-width: 100%;
}
pre code { white-space: pre; }   /* keep code on its own lines, scroll horizontally */

For prose, the opposite: let long words and URLs break instead of overflowing.

p, li, h1, h2, h3 { overflow-wrap: break-word; }

Wrap wide tables the same way you handle code, in a scroll container: <div style="overflow-x:auto">…table…</div>.

Fluid type with clamp(), not fixed desktop sizes

A heading sized for desktop is enormous on a phone and forces wrapping and overflow. clamp() scales the size smoothly between a mobile floor and a desktop ceiling with no breakpoints to juggle:

h1   { font-size: clamp(1.75rem, 4vw + 1rem, 3rem); }
body { font-size: clamp(1rem, 0.5vw + 0.9rem, 1.125rem); line-height: 1.5; }

The middle value does the scaling; the floor and ceiling keep it readable at both ends.

Mobile-first: base styles small, enhance up

Write the base styles for the narrow screen, then add @media (min-width: ...) to enhance for larger ones. Desktop-first with max-width patches is exactly how you get a layout that works on the desktop and falls apart on mobile, because the mobile case is an afterthought bolted on.

.layout { display: grid; grid-template-columns: 1fr; gap: 1rem; }   /* phone */
@media (min-width: 48rem) {
  .layout { grid-template-columns: 240px 1fr; }                     /* tablet up */
}

Check the phone width first, not last.

Sideways scroll: the rest of the causes, and the touch-scroll bug

If the page scrolls horizontally on mobile, the min-width: 0 rule above is the first thing to check. The other usual suspects:

• Set box-sizing: border-box globally. Without it, width: 100% plus any padding or border adds up to wider than the parent and overflows. This is a top cause of sideways scroll and Claude often omits it from scratch CSS:

*, *::before, *::after { box-sizing: border-box; }

• No fixed pixel widths wider than the phone. width: 800px or min-width: 600px on a 375px screen forces horizontal scroll. Use max-width, percentages, or min(800px, 100%) so the element caps at the viewport.
• Watch positioned and decorative elements. Absolutely positioned, transformed, or negative-margin elements (offset images, background blobs, things nudged with right: or translateX) stick out past the right edge and widen the scrollable area even when the layout looks fine. Constrain them, or clip on a wrapper with overflow-x: clip (preferred over hidden, it doesn't create a scroll container and so won't break position: sticky).
• Use width: 100%, not 100vw. 100vw includes the scrollbar width, so a full-width element ends up wider than the content area and scrolls the page sideways.
• Cap media, and give images dimensions: img, video, svg, canvas { max-width: 100%; height: auto; }, and set the intrinsic width/height attributes on every <img> so the browser reserves the box and the page doesn't shift or repaint when the image loads (see web-performance).
• Don't paper over it with body { overflow-x: hidden }. That hides the symptom and breaks position: sticky. Find the offender: temporarily add * { outline: 1px solid red; } (outline, not border, so it doesn't change layout) and look for the element wider than the viewport, then fix that element.
• Can't scroll a code block or editor on touch? It's almost always an ancestor with overflow: hidden clipping it, or the region has a fixed height with no overflow: auto. Give the scroll region overflow: auto and a sane max-height, and make sure no ancestor sets overflow: hidden or touch-action: none over it.

iOS Safari: when the whole page suddenly scrolls or jumps

A classic, and it's almost always one of four iOS-specific behaviors, each with a different fix:

• Scroll chaining. Drag inside a scrollable region (a modal, drawer, code block, chat list), hit its top or bottom, keep dragging, and the scroll "leaks" to the page so the whole thing moves and rubber-bands underneath. Stop it by containing the scroll on that region:

.modal-body, .drawer, .scroll-region { overscroll-behavior: contain; }

• Background scrolls under an open modal. body { overflow: hidden } does not reliably hold on iOS Safari, the page still scrolls behind the overlay. The robust lock is to fix the body and restore the scroll position on close:

// open:  remember position, freeze the body in place
const y = window.scrollY;
document.body.style.cssText = `position:fixed; top:${-y}px; left:0; right:0;`;
// close: release and jump back exactly where they were
document.body.style.cssText = "";
window.scrollTo(0, y);

• The 100vh toolbar jump. On iOS, 100vh counts the area behind Safari's address bar, so a height:100vh section is taller than the visible viewport and the page jumps as the toolbar shows and hides. Use the dynamic viewport unit instead, with a legacy fallback:

.full-height { height: 100vh; height: 100dvh; }   /* dvh tracks the real visible height */

• Tap an input and the page zooms and scrolls. iOS Safari auto-zooms when you focus an input whose font-size is under 16px, which scrolls and rescales the whole page. Give form controls at least 16px. Do not "fix" this with maximum-scale=1 or user-scalable=no, that disables pinch-zoom and hurts accessibility.

input, select, textarea { font-size: 16px; }

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This skill is built to grow. Add a rule when a real responsive failure has a stable, defensible fix.