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:

1. playwright-cli (preferred) — supports resize, named sessions, and sub-agent parallelism. If installed, run /playwright-cli first to load the full command reference.
2. Playwright MCP (mcp__plugin_playwright_playwright__*) — browser_resize for viewport changes.
3. 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:

Process:

1. Open the URL in a single browser session (height: 900px)
2. 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
3. Compare adjacent widths — identify where layout transitions occur
4. 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:

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:

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

Severity Levels

Consistent with ux-audit:

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