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