# Cdp Render Verification

> CDP Render Verification

- Skill: `ever-just/cdp-render-verification` (Agent Skill)
- Install (CLI): `npx skillmds@latest add ever-just/cdp-render-verification`
- Raw SKILL.md: https://api.skillmd.com/api/skills/ever-just/cdp-render-verification/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: ever-just (https://skillmd.com/u/ever-just)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/ever-just/cdp-render-verification

---


# CDP Render Verification

Render-verify web changes in a **real browser** before calling them done. Drives a
local headless Chrome over the DevTools Protocol with **zero dependencies** —
Node's global `WebSocket`/`fetch` and `child_process`. Screenshots are for a human
glance; the **computed-style and flow probes are the actual proof**.

**Why this exists:** shipping a visual change you only *think* is right is how you
put broken pixels or raw code in front of a user. A 40-line CDP harness lets you
assert the exact outcome: page returns 200, the intended style changed, nothing
leaked, mobile does not overflow, the flow reaches the right end state.

## When to use

- Before shipping any CSS / template / JS / content change to a live page.
- To check a **specific** property changed (e.g., an accent color is no longer
  green) — computed styles are far more reliable than eyeballing a screenshot.
- To catch **raw code leaking as text** (a `<style>`/`<script>` printed on the
  page — see [[everjust-website-customization]] Markup trap).
- To confirm **mobile** behavior (real device-metrics emulation, overflow).
- To **drive a multi-step flow** (pick date → time → fill form → review) and read
  the composed result — integration bugs a static screenshot cannot show.
- To verify a **video** autoplays/loops.

## When NOT to use

- Logic that a unit/integration test runner covers better.
- When a dedicated preview/browser MCP is connected and sufficient — though CDP
  still wins for computed-style assertions and scripted flow-driving.

---

## The harness (copy, adapt)

```js
import { spawn } from 'node:child_process';
import fs from 'node:fs';
const CHROME = "/Applications/Google Chrome.app/Contents/MacOS/Google Chrome"; // or `which google-chrome`
const port = 9333;
spawn(CHROME, ["--headless=new", `--remote-debugging-port=${port}`, "--disable-gpu",
  "--hide-scrollbars", "--no-first-run", "--autoplay-policy=no-user-gesture-required",
  "--user-data-dir=/tmp/cdp-"+port], { stdio: "ignore" });                 // fresh profile per run
const sleep = ms => new Promise(r => setTimeout(r, ms));
async function ws_url(){ for(let i=0;i<40;i++){ try{ return (await (await fetch(`http://127.0.0.1:${port}/json/version`)).json()).webSocketDebuggerUrl; }catch{ await sleep(250);} } }
const ws = new WebSocket(await ws_url()); let id=0; const pend=new Map();
ws.addEventListener("message", e=>{ const m=JSON.parse(e.data); if(m.id&&pend.has(m.id)){ pend.get(m.id)(m); pend.delete(m.id);} });
await new Promise(r=>ws.addEventListener("open", r));
const cmd=(method,params={},s)=>new Promise(res=>{ const i=++id; pend.set(i,res); ws.send(JSON.stringify({id:i,method,params,...(s?{sessionId:s}:{})})); });
const { result:{ targetId } } = await cmd("Target.createTarget", { url:"about:blank" });
const { result:{ sessionId:sess } } = await cmd("Target.attachToTarget", { targetId, flatten:true });
await cmd("Runtime.enable", {}, sess);
const ev = async x => (await cmd("Runtime.evaluate", { expression:x, returnByValue:true, awaitPromise:true }, sess)).result?.result?.value;
const shot = async name => { const r=await cmd("Page.captureScreenshot",{format:"png",captureBeyondViewport:true},sess); fs.writeFileSync(name, Buffer.from(r.result.data,"base64")); };

// DESKTOP
await cmd("Emulation.setDeviceMetricsOverride", { width:1280, height:900, deviceScaleFactor:2, mobile:false }, sess);
await cmd("Page.navigate", { url:"https://example.com/page" }, sess); await sleep(4500);   // let assets + JS settle
console.log(JSON.stringify(await ev(`JSON.stringify({
  title: document.title,
  accent: getComputedStyle(document.querySelector('.el')).color,        // computed style = the real check
  overflow: document.documentElement.scrollWidth <= window.innerWidth,  // false = horizontal overflow bug
  codeLeak: /<style|<script|&lt;/.test((document.body.innerText||'').slice(0,4000)),
  present: !!document.querySelector('.expected')
})`)));
await shot("desktop.png");

// MOBILE (real device metrics)
await cmd("Emulation.setDeviceMetricsOverride", { width:390, height:844, deviceScaleFactor:3, mobile:true }, sess);
await cmd("Page.navigate", { url:"https://example.com/page" }, sess); await sleep(4000);
console.log("mobile overflow:", await ev(`document.documentElement.scrollWidth+'/'+window.innerWidth`)); // want equal
await shot("mobile.png");
process.exit(0);
```

Run with `node harness.mjs` (Node 18+; the WebSocket global is built in on 20+/22).

## Probes that matter

- **Computed style** — `getComputedStyle(el).color` / `backgroundColor`. Assert the
  value, e.g. `!/166a0b|22,\s*106,\s*11/i.test(color)` to prove a color is gone.
- **Overflow** — `documentElement.scrollWidth` vs `window.innerWidth`; unequal on
  mobile = a horizontal-scroll bug.
- **Code leak** — `/<style|<script|&lt;/.test(body.innerText.slice(0,4000))`.
- **Video playing** — read `.readyState` (4 = enough data), `.currentTime` (>0 after
  a wait = advancing), `.paused` (false). Needs `--autoplay-policy=no-user-gesture-required`
  and a muted/`playsinline` video.
- **Console errors** — subscribe to `Runtime.exceptionThrown` messages.

## Driving a multi-step flow

Assert the *end state*, not just that a button exists:

```js
await ev(`document.querySelector('.calendar-day.available').click()`); await sleep(2500); // fetch slots
await ev(`document.querySelector('.slot').click()`); await sleep(800);
await ev(`(function(){var e=document.getElementById('email');e.value='a@b.com';e.dispatchEvent(new Event('input',{bubbles:true}));})()`);
await ev(`document.querySelector('.next').click()`); await sleep(500);
// for a native <select>: set .value then dispatch a 'change' event
console.log("composed summary:", await ev(`document.getElementById('review').textContent`));
```

This catches integration bugs (JS not wired, wrong data composed, a step not
advancing) that screenshots never reveal.

## Gotchas

- **Wait** after `Page.navigate` (~4s) — assets, fonts, and deferred module JS
  settle after DOMContentLoaded.
- Fresh `--user-data-dir` per run avoids stale state; unique port per parallel run.
- Reading an image screenshot verifies layout/vibe; for **colors/fonts/spacing**
  trust `getComputedStyle`, not the screenshot.
- To force a fresh asset after a server change, navigate again; bumping the URL or
  cache-busting the asset avoids a stale bundle.

## Related

- [[everjust-website-customization]] — the changes this verifies (and its Markup trap).
- [[web-embed-video-optimization]] — pairs with the video-playback probe.
- [[ui-ux-audit]] / [[admin-dashboard-verification]] — broader UX/feature review.