Visual Parity Audit

Visual Parity Audit

Drive two running instances of the "same" UI through matching states, take screenshots at a controlled viewport, and ship a reviewable PDF. Used to catch regressions that code review misses — typography, spacing, color, animation-entry state, or labels that drifted during a port.

When to use

• Port audits — old framework vs new framework should render pixel-close. (e.g. Express + vanilla HTML → Nuxt)
• Staging vs production — before cutover, prove no drift.
• Before client sign-off — supplier-built UI has to match designer's reference.
• After a "small" refactor — prove nothing cosmetic changed unintentionally.

Not this skill: functional BDD testing, accessibility audits, performance profiling. This is a typography-and-layout eyeball check.

Prerequisites

• Both apps reachable from this machine (one local, one live — or two URLs)
• A browser-automation MCP with viewport emulation + full-page screenshot saved to disk. In preference order: chrome-devtools (best: explicit emulate + filePath), claude-in-chrome or agent-browser (fallback: resize + saveScreenshot). If none is connected, stop and load one rather than taking skewed screenshots.
• A way to drive the state machine — either forms you can fill, or URL params that jump to each state
• latex-pdf skill available (for the PDF report)
• Basic knowledge of both apps' flows — which screens exist, how to reach each one

Process — overview

1. Lock the viewport on both apps (same width × height × DPR, mobile/desktop flag identical). Sanity-check after every screenshot.
2. Enumerate states up front — 8–15 reachable screens, numbered so file order matches reading order.
3. Drive both apps through the states. Save to docs/images/<audit>/{a,b}/<NN-slug>.png. Clear PIN/auth cookies first if relevant.
4. Write the markdown report (verdict table → regression detail → side-by-side rows → not-captured list → action items). Commit to the repo.
5. Generate the PDF from the markdown via the bundled Python script (scripts/build_parity_pdf.py) — LaTeX with \includegraphics side-by-side tables.
6. Open a PR linking the report + PDF + any regression issues.

Process — details

1. Pick the viewport — and lock it down

The #1 failure mode is a desktop screenshot vs mobile screenshot that look visually different for reasons unrelated to parity. Always emulate the same viewport on both apps, before any screenshot. For mobile UIs, use the design's target:

emulate viewport: 420x900x2,mobile,touch

The x2 is devicePixelRatio. At 420 CSS × 2 DPR, full-page screenshots are 840 px wide. Sanity-check after each capture:

sips -g pixelWidth screenshot.png

If one side is 840 and the other is 1440, you broke the emulation somewhere (usually opening a new tab resets it). Re-emulate, re-capture.

2. List the states

Write down every reachable state before you start clicking. Example for a form-based app:

01-pin (locked entry)
02-staff-mode-default
03-staff-mode-quick      (mode hint text changes)
04-staff-mode-artist     (mode hint text changes)
05-dancer-p1             (flow entry)
06-handoff
07-dancer-p2
08-quick-entry-screen    (alternate flow)
09-artist-mode-screen    (alternate flow)

Number them so file order matches presentation order. 8–15 screens is a comfortable report size. More becomes tedious to review.

Skip states that require destructive side effects (payments, production DB writes, Airtable writes) — note them in the report as "verified via code review only" with a link to the component source.

3. Drive the state machine twice — once per app

Open two tabs (or one tab used sequentially). For each state:

1. Navigate / click into the state on app A → screenshot to docs/images/capture-parity/A/{slug}.png
2. Navigate / click into the state on app B → screenshot to docs/images/capture-parity/B/{slug}.png

Save paths are stable: <repo>/docs/images/<audit-name>/{a,b}/{NN-slug}.png.

Use take_screenshot with fullPage: true and an explicit filePath so images land in the repo, not an attachment.

If an app gates state with a PIN / auth and caches the token, clear it first:

localStorage.clear();
sessionStorage.clear();
document.cookie.split(';').forEach(c => {
  const n = c.split('=')[0].trim();
  document.cookie = `${n}=;expires=Thu,01 Jan 1970 00:00:00 GMT;path=/`;
});

4. Write the markdown report first

Before the PDF, produce <repo>/docs/<audit-name>-<YYYY-MM-DD>.md with:

• Verdict table — 1 row per aspect, ✓ / ✗ / ~ with one-line note
• Regression detail — if you find a drift that's actionable, write it up as a standalone section with the CSS / markup diff, not hidden in a table
• Side-by-side rows — one section per screen, markdown table with 2 cells holding ![](path/to/a.png) and ![](path/to/b.png)
• Screens not captured — explicit list with reason (transient, requires side effect, etc.)
• Action items — P0/P1/P2/P3 table with issue numbers if you open them

Commit this markdown as the canonical source. GitHub renders it fine for quick review.

5. Generate the PDF

The markdown is for engineering review; the PDF is for stakeholders who don't open GitHub. Use the bundled scripts/build_parity_pdf.py — a self-contained Python script that emits LaTeX and compiles with Tectonic. No runtime deps beyond python3 + tectonic (brew install tectonic).

Workflow:

1. Copy scripts/build_parity_pdf.py to a working location (e.g. ~/Local/<org>/<project>/build-<audit>.py so it lives next to the PDF output).
2. Edit the config block at the top: AUDIT_NAME, IMG_DIR, OUTPUT_PDF, TITLE, SUBTITLE, HEADLINE_CALLOUT, and populate the ROWS, VERDICT, NOT_CAPTURED, ACTIONS lists.
3. Run python3 build-<audit>.py. The script writes a .tex sidecar and the .pdf next to it.

The template is intentionally skeletal — it handles LaTeX preamble, fonts, colors, header/footer, half-width image cells with keepaspectratio, unnumbered \section*, and a red-bordered tcolorbox for the headline callout. You only supply the audit-specific content.

Key design decisions baked into the script:

• height=0.52\textheight,keepaspectratio stops tall mobile screenshots from pushing rows off the page
• \section* (unnumbered) keeps the TOC clean
• Row labels are "Reference (A) / Under audit (B)" — generic so the PDF reads right for both "prototype vs port" and "staging vs production" audits
• Saves to ~/Local/<org>/<project>/ per the media-path convention, not in the repo

If you need to deviate (full-width images, three-column comparison, etc.), edit the script directly — it's <200 lines.

6. Open a PR

One PR with:

• Markdown report
• Screenshot directory
• In the PR body: the same verdict table, a link to the PDF artefact, and a callout of any regression issues opened

Label with the relevant flow/area label and documentation. Milestone it if the audit is part of a sprint.

Gotchas

• Viewport drift between apps — catch with sips -g pixelWidth sanity check. If both sides aren't the same width, redo.
• Token / cookie persistence — both apps may remember a prior session; clear storage before capturing the locked state (e.g. PIN screen).
• Non-deterministic state — counters, ads, seed data. Either seed the same state on both sides or acknowledge the difference in the row notes.
• Side effects on the "under audit" side — never click Save / Submit on production during an audit. Use the "not captured" list instead.
• Scroll-dependent screenshots — use fullPage: true always, not viewport-only.
• Font smoothing / retina — set deviceScaleFactor: 2 on both sides for readable text at phone aspect ratios.

Anti-patterns

• Don't write the markdown after the PDF. The markdown is the canonical version-controlled record; the PDF is a rendering.
• Don't let the audit conclude "looks good." The value is catching drift — if you find none, say so explicitly and note what you did verify.
• Don't skip the "not captured" list. Auditors reading the PDF assume absence means aligned; being explicit about what wasn't checked is the honest move.
• Don't trust a code-level audit to have surfaced everything. The visual pass exists precisely because code audits misread stylesheets (I learned this one the hard way — labels that looked "too big" in an earlier port audit were actually the designer's intent).

Output layout

<repo>/docs/
  <audit-name>-<YYYY-MM-DD>.md     # canonical report (version-controlled)
  images/<audit-name>/
    a/NN-slug.png                  # reference side
    b/NN-slug.png                  # under-audit side

~/Local/<org>/<project>/
  <audit-name>-<YYYY-MM-DD>.pdf    # rendered artefact for stakeholders
  <audit-name>-<YYYY-MM-DD>.tex    # sidecar, regenerable
  build-<audit>.py                 # copy of scripts/build_parity_pdf.py with this audit's config

Bundled resources

• scripts/build_parity_pdf.py — self-contained LaTeX builder. Copy, edit config block, run. No pip deps; needs tectonic installed.