Verify Visually

Rendering and looking

The checks are static readers. They cannot see a stretched grid, a collapsed flex item, a truncated label, a card wrapping 3+1, or text that has become unreadable. A clean run means "nothing is structurally broken", not "it looks right" — so for anything that changes what a reader sees, render it.

.github/scripts/shot.sh index.html                          # light, 1400px
.github/scripts/shot.sh index.html --dark
.github/scripts/shot.sh 'index.html#ingress2gateway'        # a specific view
.github/scripts/shot.sh ingress-nginx-migration.html --width 900
.github/scripts/shot.sh index.html --measure '.sidebar-link-name'

Then read the PNG. Writing the file is not verification.

Four facts that make a correct command look broken

1. Both pages are hash-routed SPAs. index.html has five .product-page panes, four of them hidden="until-found"; the migration tool has three .tool-page. A screenshot with no hash shows the home view, so a change to the ingress2gateway section or the reference tables will not appear and you will conclude, wrongly, that it did not apply. Deep-link: index.html#ingress2gateway, ingress-nginx-migration.html#reference. The hash applies on first load with no interaction.

2. Headless Chrome renders the dark theme by default here. It reports prefers-color-scheme: dark, and the inline flash-prevention script honours that. --force-dark-mode does not drive this site — dark mode is a class toggled from localStorage.darkMode. shot.sh seeds that key ahead of the inline script, which is the only reliable way to choose a theme. If you render by hand and get a dark page you did not ask for, this is why.

3. --screenshot captures the viewport, not the page — and it captures it at scroll position zero. Those two together are nastier than either alone: deep- link to #mappings, the document scrolls, and a fixed 1200px capture of a 4,751px page comes back almost entirely blank, because the content has moved out of the captured band. It looks like the page failed to render.

shot.sh handles this by measuring the laid-out document first and sizing the window to it, capped at 8000px (it says so when it caps). Pass --height to override. If you render by hand, size the window to the document or you will be looking at empty space.

4. Widths below 500px are a lie on macOS. The viewport is clamped, so 320, 375 and 400 all render at 500 and the PNG is cropped — which reads exactly like horizontal overflow that is not there. For phone widths, measure instead of looking, or trust the 900/600px breakpoints in the CSS.

Measuring beats looking

For anything with a number in it — does this label fit, does this row overflow, is this touch target big enough — take a measurement rather than squinting at an image. --measure takes a CSS selector and prints JSON: each node's box, its text, whether it is clipped (scrollWidth > width), the viewport, and whether the document overflows horizontally.

That is how the sidebar label question was settled: "Phased migration strategy" measures 150.5px against ~226px of usable rail, so it fits; "Ingress NGINX config analyzer" would be ~285px, so it does not, and the sidebar shortens it. Both facts are now in AGENTS.md because they were measured, not guessed.

Structured output also diffs. For a change that should not alter layout, measure before and after and compare the JSON — far more reliable than comparing two images by eye.

When you cannot render

shot.sh exits 3 with NO-BROWSER: when it finds no Chrome or Chromium. That is not a reason to skip the step quietly: say in your summary that you could not render, and fall back to diffing structured output against a pristine worktree. Reporting green checks as visual verification is the specific failure this exists to prevent — they cannot see layout, which is the whole point.

Other things worth knowing

• python3 -m http.server from the repo root is a fine manual preview. Every asset path is relative, so file:// also mostly works; a server is closer to production.
• index.js fetches live release versions from the GitHub API, so a rendered page shows current upstream versions even when the files are stale. Do not read a screenshot as evidence that the pinned versions are right — that is what check-versions.py is for. Pin the network with --host-resolver-rules="MAP * ~NOTFOUND, EXCLUDE 127.0.0.1" for anything you intend to diff.
• Chrome does not always exit after writing its artifact; shot.sh wraps it in timeout, and the PNG is complete even when the wrapper fires.
• There is no PR preview environment. Pages serves main only. Verify locally before pushing, because pushing to main is deploying.