Web Application Testing Validator

Validate that a web application renders correctly, responds to user interaction, and produces no runtime errors by running Playwright-based checks from the terminal.

Overview

This skill validates frontend behavior of local or staging web applications through automated Playwright scripts. Instead of writing full test suites, it runs targeted checks that produce a clear pass/fail result for each validation rule.

What it validates:

• Page loads without JavaScript errors
• Critical elements are present and visible after render
• Form submissions reach the expected success state
• Console output contains no uncaught exceptions or failed network calls
• Layout responds correctly to viewport changes

Standards enforced:

• Zero uncaught JavaScript errors in the console
• All expected DOM elements present after networkidle
• HTTP response codes in the 2xx range for primary resources
• Forms produce the expected post-submission state

Prerequisites:

• Python 3.8+ with playwright package installed (pip install playwright && playwright install chromium)
• The bundled scripts/with_server.py helper for managing server lifecycle
• Target application source or a running server URL

Rules

Rule 1: Page Load Success

Description: Navigates to the target URL and waits for networkidle. Passes when the page reaches a loaded state within the timeout.

Passes when: page.wait_for_load_state('networkidle') completes within 30 seconds and page.title() returns a non-empty string.

Fails when: Navigation times out or returns an HTTP error status (4xx/5xx).

Severity: Critical

Rule 2: Zero Console Errors

Description: Captures all browser console messages during page load and interaction. Fails when any error-level message appears.

Passes when: No console.error messages are emitted during the check window.

Fails when: One or more error-level console messages appear. Warning-level messages produce a notice but do not fail the check.

Severity: High

Rule 3: Element Presence

Description: Verifies that a list of expected selectors exist in the rendered DOM after networkidle.

Passes when: Every selector in the checklist resolves to at least one visible element via page.locator(selector).first.is_visible().

Fails when: Any selector returns zero matches or all matches are hidden.

Severity: High

Rule 4: Form Submission

Description: Fills a form with test data, submits it, and verifies the post-submission state.

Passes when: After submission the page contains the expected success indicator (text, URL change, or element).

Fails when: The success indicator is absent after a 10-second wait, or a console error occurs during submission.

Severity: Medium

Rule 5: Responsive Layout

Description: Resizes the viewport to mobile (375x812), tablet (768x1024), and desktop (1920x1080) breakpoints, taking a screenshot at each.

Passes when: The page renders without horizontal overflow at every breakpoint (document.documentElement.scrollWidth <= window.innerWidth).

Fails when: Horizontal overflow is detected at any breakpoint.

Severity: Medium

Rule 6: Network Health

Description: Monitors all network requests during page load. Flags failed requests (status >= 400) and mixed-content warnings.

Passes when: All XHR/Fetch requests return 2xx status codes and no mixed-content requests are detected.

Fails when: Any primary resource returns 4xx/5xx or the page triggers mixed-content warnings.

Severity: High

Thresholds

Pass/Fail Criteria

Automatic PASS

All of the following must be true:

• Page loads within timeout with a non-empty title
• Zero error-level console messages
• All checklist selectors resolve to visible elements
• No network requests with status >= 400

Automatic FAIL

Any of the following triggers failure:

• Page navigation times out or returns HTTP error
• Uncaught JavaScript exception in the console
• A critical-severity selector is missing from the DOM
• Server process fails to start (when using with_server.py)

Conditional PASS

May pass with warnings if:

• Console contains warning-level messages only
• Non-critical selectors are missing but critical selectors are present
• Network requests to third-party domains fail (analytics, CDN) but primary resources load

Workflow

Step 1: Start the Server

If the application is not already running, use the bundled helper:

python scripts/with_server.py --server "npm run dev" --port 5173 -- python your_checks.py

For multiple servers (backend + frontend):

python scripts/with_server.py \
  --server "cd backend && python manage.py runserver 0.0.0.0:8000" --port 8000 \
  --server "cd frontend && npm run dev" --port 5173 \
  -- python your_checks.py

Always run python scripts/with_server.py --help first to see all options.

Step 2: Run Reconnaissance

Before writing checks, inspect the rendered page:

from playwright.sync_api import sync_playwright

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True)
    page = browser.new_page()
    page.goto("http://localhost:5173")
    page.wait_for_load_state("networkidle")

    # Discover elements
    buttons = page.locator("button").all()
    print(f"Buttons: {len(buttons)}")
    for btn in buttons:
        print(f"  - {btn.inner_text()}")

    inputs = page.locator("input, textarea, select").all()
    print(f"Input fields: {len(inputs)}")
    for inp in inputs:
        name = inp.get_attribute("name") or inp.get_attribute("id") or "[unnamed]"
        print(f"  - {name} ({inp.get_attribute('type') or 'text'})")

    page.screenshot(path="/tmp/recon.png", full_page=True)
    browser.close()

Step 3: Build the Selector Checklist

From reconnaissance results, list the selectors that must be present:

REQUIRED_SELECTORS = [
    ("h1", "Page heading"),
    ("nav", "Navigation bar"),
    ("button[type='submit']", "Submit button"),
    ("#main-content", "Main content area"),
]

Step 4: Execute Validation

Run checks against each rule. The script below combines all rules:

from playwright.sync_api import sync_playwright
import sys

URL = "http://localhost:5173"
REQUIRED_SELECTORS = [("h1", "heading"), ("nav", "navigation")]
errors = []
warnings = []
console_errors = []

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True)
    page = browser.new_page()
    page.on("console", lambda msg: (
        console_errors.append(msg.text) if msg.type == "error"
        else warnings.append(msg.text) if msg.type == "warning"
        else None
    ))

    # Rule 1: Page load
    try:
        response = page.goto(URL, wait_until="networkidle", timeout=30000)
        if response and response.status >= 400:
            errors.append(f"FAIL Rule 1: HTTP {response.status}")
        else:
            title = page.title()
            print(f"PASS Rule 1: Page loaded (title: {title})")
    except Exception as e:
        errors.append(f"FAIL Rule 1: {e}")

    # Rule 2: Console errors (checked after all interactions)
    # Rule 3: Element presence
    for selector, label in REQUIRED_SELECTORS:
        loc = page.locator(selector).first
        if loc.is_visible():
            print(f"PASS Rule 3: '{label}' visible")
        else:
            errors.append(f"FAIL Rule 3: '{label}' ({selector}) not visible")

    # Rule 5: Responsive check
    for width, height, label in [(375, 812, "mobile"), (1920, 1080, "desktop")]:
        page.set_viewport_size({"width": width, "height": height})
        page.wait_for_timeout(500)
        overflow = page.evaluate(
            "document.documentElement.scrollWidth > window.innerWidth"
        )
        if overflow:
            errors.append(f"FAIL Rule 5: Horizontal overflow at {label} ({width}x{height})")
        else:
            print(f"PASS Rule 5: No overflow at {label}")

    browser.close()

# Rule 2: Console error summary
if console_errors:
    errors.append(f"FAIL Rule 2: {len(console_errors)} console error(s)")
    for e in console_errors[:3]:
        print(f"  console.error: {e}")
else:
    print("PASS Rule 2: Zero console errors")

if warnings:
    print(f"WARNING: {len(warnings)} console warning(s)")

# Verdict
print()
if errors:
    print("VALIDATION: FAILED")
    for e in errors:
        print(f"  {e}")
    sys.exit(1)
else:
    print("VALIDATION: PASSED")
    sys.exit(0)

Step 5: Review Results

After running the validation script:

• PASSED: All rules met. The application is ready for deployment.
• FAILED: Review the error list. Each failure references the rule number and specific issue.
• WARNINGS: Console warnings or non-critical selector misses. Investigate but not blocking.

Screenshots saved during validation provide visual evidence for review.

Examples

Example 1: Validate a React Development Server

User: Check if the React app at localhost:5173 works correctly
Assistant:
  1. python scripts/with_server.py --help
  2. python scripts/with_server.py --server "npm run dev" --port 5173 \
       -- python validate_app.py
  3. Results:
     PASS Rule 1: Page loaded (title: My App)
     PASS Rule 2: Zero console errors
     PASS Rule 3: 'heading' visible
     PASS Rule 3: 'navigation' visible
     PASS Rule 5: No overflow at mobile
     PASS Rule 5: No overflow at desktop

     VALIDATION: PASSED

Example 2: Detect Broken API Calls

User: The dashboard page shows a blank table after the last deploy
Assistant:
  1. Navigate to http://localhost:3000/dashboard
  2. Capture console output and network requests
  3. Results:
     PASS Rule 1: Page loaded (title: Dashboard)
     FAIL Rule 2: 1 console error(s)
       console.error: Uncaught TypeError: Cannot read property 'map' of undefined
     FAIL Rule 6: GET /api/users returned 500
     FAIL Rule 3: '#user-table tbody tr' not visible

     VALIDATION: FAILED
       FAIL Rule 2: 1 console error(s)
       FAIL Rule 3: '#user-table tbody tr' not visible
       FAIL Rule 6: GET /api/users returned 500

     Root cause: The /api/users endpoint returns 500, causing the
     frontend to receive undefined instead of an array.

Example 3: Responsive Layout Check

User: Verify the landing page looks good on mobile
Assistant:
  1. Navigate to http://localhost:5173
  2. Resize to 375x812 (iPhone 12)
  3. Results:
     PASS Rule 1: Page loaded
     PASS Rule 2: Zero console errors
     FAIL Rule 5: Horizontal overflow at mobile (375x812)
       scrollWidth=412 > viewportWidth=375

     VALIDATION: FAILED
       FAIL Rule 5: Horizontal overflow at mobile (375x812)

     The page has 37px of horizontal overflow on mobile.
     Check for elements with fixed widths wider than 375px.

Exit Codes

Best Practices

• Always wait for networkidle before inspecting the DOM on dynamic applications
• Run --help on bundled scripts before reading their source code
• Use page.screenshot() to capture visual state at each check point
• Prefer CSS selectors and ARIA roles over XPath for element targeting
• Run checks in headless mode for CI; add headless=False for debugging
• Close the browser in a finally block to prevent orphan processes
• Store screenshots under /tmp/webapp-test/ or a project-specific output directory

References

Load these on demand for deeper detail:

• Playwright patterns: references/playwright-patterns.md -- selector strategies, wait conditions, and common recipes
• Testing checklist: references/testing-checklist.md -- comprehensive list of frontend validation items by category
• Server management: references/server-management.md -- advanced with_server.py usage and troubleshooting