Web Application Testing Skill

Web Application Testing

To test local web applications, write native Python Playwright scripts.

Helper Scripts Available:

scripts/with_server.py - Manages server lifecycle (supports multiple servers)

Always run scripts with --help first to see usage. DO NOT read the source until you try running the script first and find that a customized solution is abslutely necessary. These scripts can be very large and thus pollute your context window. They exist to be called directly as black-box scripts rather than ingested into your context window.

Decision Tree: Choosing Your Approach

User task → Is it static HTML?
    ├─ Yes → Read HTML file directly to identify selectors
    │         ├─ Success → Write Playwright script using selectors
    │         └─ Fails/Incomplete → Treat as dynamic (below)
    │
    └─ No (dynamic webapp) → Is the server already running?
        ├─ No → Run: python scripts/with_server.py --help
        │        Then use the helper + write simplified Playwright script
        │
        └─ Yes → Reconnaissance-then-action:
            1. Navigate and wait for networkidle
            2. Take screenshot or inspect DOM
            3. Identify selectors from rendered state
            4. Execute actions with discovered selectors

Example: Using with_server.py

To start a server, run --help first, then use the helper:

Single server:

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

Multiple servers (e.g., backend + frontend):

python scripts/with_server.py \
  --server "cd backend && python server.py" --port 3000 \
  --server "cd frontend && npm run dev" --port 5173 \
  -- python your_automation.py

To create an automation script, include only Playwright logic (servers are managed automatically):

from playwright.sync_api import sync_playwright

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True) # Always launch chromium in headless mode
    page = browser.new_page()
    page.goto('http://localhost:5173') # Server already running and ready
    page.wait_for_load_state('networkidle') # CRITICAL: Wait for JS to execute
    # ... your automation logic
    browser.close()

Reconnaissance-Then-Action Pattern

Inspect rendered DOM:

page.screenshot(path='/tmp/inspect.png', full_page=True)
content = page.content()
page.locator('button').all()

Identify selectors from inspection results
Execute actions using discovered selectors

Common Pitfall

❌ Don't inspect the DOM before waiting for networkidle on dynamic apps ✅ Do wait for page.wait_for_load_state('networkidle') before inspection

Debugging CSS Layout Bugs (sticky, scroll containers, height chains)

A screenshot or a plausible-sounding root cause is not sufficient confirmation for a layout bug (sticky positioning, scroll containers, flex/grid height chains). These bugs frequently have root causes stacked across the DOM ancestor chain — a locally correct fix (e.g. position: sticky on the target element) can still fail because a parent or the shared app shell constrains it (e.g. align-items: start collapsing the grid item's height, or a shell using min-height: 100vh instead of height: 100vh so no ancestor ever actually becomes a scrolling container).

Protocol:

Pin down the exact failing measurement before attempting a fix — not "does it look right," but a specific getBoundingClientRect() / computed-style / scrollTop value at a specific scroll position that demonstrates the bug. Example:
```
page.evaluate("window.scrollTo(0, 1200)")
rect = page.locator(".rightColumn").evaluate("el => el.getBoundingClientRect()")
scroll_top = page.locator(".content").evaluate("el => el.scrollTop")
```
Re-run that identical measurement after every fix attempt. Only declare the bug fixed when that specific measurement now passes — not when a new theory sounds more precise than the last one, and not from a screenshot alone.
Expect to walk up the ancestor chain. If a fix is syntactically correct but the measurement still fails, the real constraint is usually one level higher (grid/flex alignment → the element's own overflow container → a shared shell's height model). Don't stop at the first plausible explanation; keep measuring until the failure condition itself is gone.

Best Practices

Use bundled scripts as black boxes - To accomplish a task, consider whether one of the scripts available in scripts/ can help. These scripts handle common, complex workflows reliably without cluttering the context window. Use --help to see usage, then invoke directly.
Use sync_playwright() for synchronous scripts
Always close the browser when done
Use descriptive selectors: text=, role=, CSS selectors, or IDs
Add appropriate waits: page.wait_for_selector() or page.wait_for_timeout()

Rendering HTML to PNG

Use uv run --with playwright — no venv setup required. Install the browser once with uv run --with playwright python3 -m playwright install chromium.

from playwright.sync_api import sync_playwright
import os

html_path = os.path.abspath('path/to/file.html')
out_path = os.path.abspath('path/to/output.png')

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True)
    page = browser.new_page(device_scale_factor=4)   # 4–5x for sharp output
    page.set_viewport_size({'width': 1400, 'height': 900})
    page.goto(f'file://{html_path}')
    page.wait_for_load_state('networkidle')
    # Screenshot a specific element to avoid body padding:
    page.locator('.my-container').screenshot(path=out_path)
    # Or full page:
    # page.screenshot(path=out_path, full_page=True)
    browser.close()

Key decisions:

device_scale_factor=4 or 5 — required for sharp text; 1x (default) looks blurry
Element screenshot (locator.screenshot()) crops to content, avoids body padding
full_page=True captures everything but includes body margins — use only if you want the full document

Reference Files

examples/ - Examples showing common patterns:
- element_discovery.py - Discovering buttons, links, and inputs on a page
- static_html_automation.py - Using file:// URLs for local HTML
- console_logging.py - Capturing console logs during automation

Agent Skills: Web Application Testing

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Skill Files