Responsiveness Check

Test how a website's layout responds to viewport width changes. Resizes through breakpoints in a single browser session, screenshots each width, compares adjacent sizes, and reports where layouts break.

What this tests: Layout responsiveness — overflow, stacking, navigation transitions, content reflow.

What this does NOT test: General accessibility (ARIA, semantic HTML, heading hierarchy, colour contrast). Those don't vary by viewport width — use the ux-audit skill instead.

Browser Tool Priority

Before starting, detect available browser tools:

• playwright-cli (preferred) — supports resize, named sessions, and sub-agent parallelism. If installed, run /playwright-cli first to load the full command reference.

• Playwright MCP (mcp__plugin_playwright_playwright__* ) — browser_resize for viewport changes.

• Chrome MCP (mcp__claude-in-chrome__* ) — resize_window for viewport changes. Uses the user's logged-in Chrome session.

If none are available, inform the user and suggest installing playwright-cli or Playwright MCP.

Operating Modes

Mode 1: Standard Check

When: "check responsive", "responsiveness check", "test breakpoints"

Test 8 key breakpoints that cover the device spectrum:

Width Device Context

320px Small phone (iPhone SE)

375px Standard phone (iPhone 14)

768px Tablet portrait (iPad)

1024px Tablet landscape / small laptop

1280px Laptop

1440px Desktop

1920px Full HD

2560px Ultra-wide / 4K

Process:

• Open the URL in a single browser session (height: 900px)

• Start at 320px. For each breakpoint width: a. Resize the viewport b. Wait briefly for CSS reflow (layout transition) c. Screenshot the above-fold area d. If the page has significant below-fold content, scroll and screenshot e. Run the 8 layout checks (see matrix below) f. Note any issues with severity

• Compare adjacent widths — identify where layout transitions occur

• Write the report

Mode 2: Sweep

When: "responsive sweep", "sweep all breakpoints", "find where it breaks"

Test every 160px from 320 to 2560 (15 widths total). Same single-session approach as Standard — just more data points. This is the mode for finding the exact width where a layout breaks.

Widths: 320, 480, 640, 800, 960, 1120, 1280, 1440, 1600, 1760, 1920, 2080, 2240, 2400, 2560

Briefly confirm before starting sweep mode (15 screenshots is a meaningful session).

Mode 3: Targeted Range

When: "check between 768 and 1024", "test tablet breakpoints", "focus on mobile widths"

Test a user-specified range at 80px increments. Use when a known trouble zone needs detailed investigation.

Example: "check between 768 and 1024" tests: 768, 848, 928, 1008 (plus 1024 as endpoint).

Multi-URL

When testing multiple URLs (e.g., "check the homepage, about page, and contact page"):

• Launch parallel sub-agents, one per URL (not per breakpoint)

• Each sub-agent runs a standard check on its URL in its own named session

• Combine results into a single report

Sub-agent pattern (playwright-cli)

playwright-cli -s=page1 open https://example.com/ & playwright-cli -s=page2 open https://example.com/about &

Layout Check Matrix

These 8 checks target issues that actually vary by viewport width:

Check What to Look For

1 Horizontal overflow Content wider than viewport — horizontal scrollbar appears, elements cut off

2 Text overflow Text truncated mid-word, overlapping adjacent elements, font size unreadable (< 12px)

3 Navigation transition Hamburger menu appears/disappears at correct width, no "broken" state between modes

4 Content stacking Multi-column layouts stack to single column in logical reading order on narrow widths

5 Image/media scaling Images overflow container, distorted aspect ratios, missing responsive sizing

6 Touch targets Interactive elements < 44px on mobile widths (< 768px) — buttons, links, form inputs

7 Whitespace balance Too cramped on mobile (no breathing room), too sparse on wide screens (content lost in space)

8 CTA visibility Primary call-to-action visible above the fold at each width without scrolling

Transition Detection

The unique value of this skill is finding where layout transitions happen and whether they're clean.

When comparing screenshots at adjacent widths, flag any width where:

• Column count changes (3-col → 2-col → 1-col grid)

• Navigation mode switches (full nav → hamburger, or vice versa)

• Sidebar appears/disappears (content width jumps)

• Grid reflows (cards wrap to next row)

Report the exact width range where each transition occurs:

Transition From To Width Range

Nav: hamburger → full 768px 1024px Switches at ~960px

Grid: 1-col → 2-col 640px 768px Reflows at ~700px

Sidebar appears 1024px 1280px Shows at ~1100px

This tells the developer exactly where to set (or fix) their CSS breakpoints.

Severity Levels

Consistent with ux-audit:

Severity Meaning

Critical Layout is broken — content unreadable, navigation inaccessible, page unusable

High Significant layout issue — major overflow, key content hidden, broken transition

Medium Noticeable but usable — awkward spacing, minor overflow, suboptimal stacking order

Low Polish — whitespace tweaks, slight alignment issues, minor touch target shortfalls

Autonomy Rules

• Just do it: Resize viewport, take screenshots, analyse layout, compare widths

• Brief confirmation: Before sweep mode (15 viewports), before testing 4+ URLs in parallel

• Ask first: Before interacting with forms or clicking through authentication flows

Report Output

Write report to docs/responsiveness-check-YYYY-MM-DD.md (or inline for single-page quick checks).

See references/report-template.md for the report structure.

Reference Files

When Read

Looking up breakpoint details and trouble zones references/breakpoints.md

Writing the responsiveness report references/report-template.md