render-pulse

Find the CSS and JavaScript that block first paint in your built site, with the defer, async, media or critical-CSS fix for each.

Nothing is painted until the browser has downloaded and parsed every render-blocking stylesheet and synchronous script in <head>. One forgotten <script src> without defer, a 200 kB framework stylesheet or an @import chain can add seconds to First Contentful Paint and LCP on mobile. render-pulse reads your static build output, works out which resources are actually on the critical path (respecting defer, async, type=module, media, disabled and alternate stylesheets) and reports each one with a concrete fix.

CLI, library and GitHub Action. Node.js 18+, zero runtime dependencies. Works on any static build output (Astro, Next.js output: 'export', Eleventy, Hugo, Jekyll, Docusaurus, VitePress, plain HTML) and runs offline: point it at the folder your build writes. By Louis Wright.

Features

Scripts: classic <script src> in <head> without defer/async (high), synchronous external scripts in the middle of <body>, document.write(), scripts placed before stylesheets.
Stylesheets: gzip size of all render-blocking CSS per page against a budget (default 50 kB), too many blocking stylesheets, @import in CSS files or inline styles, print stylesheets without media="print", and a hint to inline critical CSS when blocking CSS exceeds the 14 kB first round trip.
Third parties: render-blocking resources from other origins without <link rel="preconnect">.
Knows the async patterns: media="print" onload="this.media='all'", module scripts, JSON-LD and template scripts are not reported.

Scope

render-pulse inspects the HTML and CSS on disk; it does not run a browser. Font loading has its own tool (font-pulse), JavaScript size is bundle-pulse, and lab timings are vitals-pulse.

Rules

Rule	Default severity	What it means
`render-blocking-script`	high	Classic <script src> in <head> without defer or async blocks rendering
`render-blocking-css-heavy`	medium	Render-blocking CSS is large (inline the critical CSS, load the rest later)
`render-css-import`	medium	@import in render-blocking CSS serialises downloads
`render-third-party-blocking`	medium	Render-blocking resource from another origin without preconnect
`render-document-write`	medium	document.write() in a script delays parsing and can block on network
`render-parser-blocking-body`	low	Synchronous external <script> in <body> blocks parsing of everything after it
`render-too-many-stylesheets`	low	Many render-blocking stylesheets
`render-print-css-blocking`	low	Print/alternate stylesheet without a matching media attribute blocks rendering
`render-large-inline`	low	Very large inline <style> or <script> in <head>
`render-sync-before-css`	low	Synchronous script placed before stylesheets delays CSS discovery
`render-no-critical-css`	info	Blocking CSS over 14 KB and no inline critical CSS
`allowlist-expired`	medium	An allowlist entry is past its `expires` date
`allowlist-unused`	info	An allowlist entry matched nothing

Install and run

npx render-pulse dist
npx render-pulse public --max-css-kb 30 --fail-on medium
npx render-pulse _site --format sarif > render.sarif

Pages are mapped to URLs the way static hosts serve them: index.html → /, blog/index.html → /blog/, about.html → /about.html (or /about with --clean-urls). Pass --base-url with your public URL, or let it be inferred from canonical / og:url tags. Meta-refresh redirect stubs and 404.html are skipped. X-Robots-Tag and Link headers are read from Netlify/Cloudflare _headers and vercel.json.

Sample output

render-pulse — https://example.com
1 page(s) at https://example.com

HIGH     index.html:2 /: <script src="/js/jquery.js"> (0.0 KB gzip) in <head> blocks rendering [render-blocking-script]
HIGH     index.html:7 /: <script src="https://tags.example.org/t.js"> in <head> blocks rendering [render-blocking-script]
MEDIUM   css/all.css css/all.css uses @import [render-css-import]
MEDIUM   index.html /: render-blocking resource from https://fonts.example.org without <link rel="preconnect"> [render-third-party-blocking]
MEDIUM   index.html /: render-blocking resource from https://tags.example.org without <link rel="preconnect"> [render-third-party-blocking]
MEDIUM   index.html /: 56.2 KB gzip (195.4 KB raw) of render-blocking CSS (budget 50.0 KB) [render-blocking-css-heavy]

Options

render-pulse 1.0.0 — find render-blocking CSS and JS in your built site

Usage: render-pulse [site-dir] [options]

Site
      --base-url <url>        Public URL of the site, e.g. https://example.com (default: inferred from canonical/og:url)
      --include <globs>       Only check these pages (file or URL path globs, comma-separated)
      --exclude <globs>       Skip these pages
      --clean-urls            Pages are served without .html (about.html -> /about)
Render
      --max-css-kb <n>         Render-blocking CSS budget per page, gzip KB (default 50)

Common
  -c, --config <file>     JSON config (default: .render-pulse.json or "renderPulse" in package.json)
  -f, --format <fmt>      text (default) | json | sarif | markdown
      --json <file>       Also write a JSON report
      --sarif <file>      Also write SARIF 2.1.0
      --markdown <file>   Also write Markdown (e.g. for a job summary)
      --fail-on <sev>     critical | high | medium | low | info | none (default: high)
      --list-rules        Print rules and default severities
      --no-fixes          Hide fix suggestions in text output
      --no-color          Disable colours
  -v, --version / -h, --help

Exit codes: 0 = passed, 1 = findings at or above --fail-on, 2 = usage, config or read error.

Configuration and allowlist

Options can live in .render-pulse.json (or a "renderPulse" key in package.json, or --config <file>):

{
  "failOn": "medium",
  "severity": {
    "render-blocking-script": "low"
  },
  "allowlist": [
    {
      "rule": "render-blocking-script",
      "subject": "/js/consent-loader.js",
      "reason": "Consent manager must run before other tags",
      "expires": "2027-03-31"
    }
  ],
  "maxCss": 40960,
  "maxStylesheets": 2,
  "maxInline": 30720,
  "exclude": [
    "/legacy/**"
  ]
}

severity changes a rule's severity, or turns it "off".
allowlist entries suppress matching findings. Each needs a reason and matches on any combination of rule, file, subject (globs with *) and match (a substring of the message). Add expires (YYYY-MM-DD) so the exception is time-boxed: once it lapses the finding comes back along with an allowlist-expired finding (medium). Entries that no longer match anything are reported as allowlist-unused (info).

Config can also set baseUrl, include, exclude and cleanUrls. The config file is looked up in the site directory first, then in the current directory, so .render-pulse.json can live at your repository root. Other keys: maxCss (bytes gzip), maxStylesheets (default 3), maxInline (default 50 kB), plus baseUrl, include, exclude and cleanUrls.

Output and exit codes

--format text (default), json, sarif (SARIF 2.1.0 for GitHub code scanning) or markdown (for job summaries and PR comments). --json, --sarif and --markdown write extra report files in the same run.
--fail-on <severity> sets the threshold (critical, high, medium, low, info, none; default high).
Exit codes: 0 passed, 1 findings at or above --fail-on, 2 usage, config or read error.

GitHub Action

permissions:
  contents: read
  security-events: write   # only needed for the SARIF upload
jobs:
  render-pulse:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4
        with: { persist-credentials: false }
      - run: npm ci && npm run build   # produce the static site first
      - uses: MrLouisWright/render-pulse@v1
        with:
          path: dist
          fail-on: high
          sarif: render-pulse.sarif
      - uses: github/codeql-action/upload-sarif@v3
        if: always()
        with: { sarif_file: render-pulse.sarif }

Input	Description	Default
`path`	Built site directory (e.g. dist, public, _site)	`.`
`base-url`	Public URL of the site, e.g. https://example.com
`fail-on`	Failure threshold	`high`
`config`	Config file path
`sarif` / `json`	Write SARIF / JSON report to this path
`job-summary`	Add the Markdown report to the job summary	`true`
`args`	Extra CLI arguments

The action runs the bundled CLI with Node 20; it has no dependencies to install. Pin it to a commit SHA if your policy requires it.

Library

import { check, blocksRender } from "render-pulse";
blocksRender({ rel: "stylesheet", media: "print" }); // false
const report = check("dist", { maxCss: 30 * 1024 });

Types are included.

Requirements

Node.js 18 or newer. Zero runtime dependencies.

Support

If this project is useful to you, you can support its development with a donation: Buy me a coffee. It helps keep these tools maintained, documented and free for everyone.

Credit (optional)

Free to use under MIT, no attribution required. If render-pulse helps you, a link back is appreciated:

<a href="https://louiswright.co.uk">Built with render-pulse by Louis Wright</a>

[![Built with render-pulse by Louis Wright](https://img.shields.io/badge/built%20with-render--pulse-blue)](https://louiswright.co.uk)