accounted/.agents/skills/vercel-react-best-practices/rules/rendering-script-defer-async.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Use defer or async on Script Tags	HIGH	eliminates render-blocking	rendering, script, defer, async, performance

Use defer or async on Script Tags

Impact: HIGH (eliminates render-blocking)

Script tags without defer or async block HTML parsing while the script downloads and executes. This delays First Contentful Paint and Time to Interactive.

• defer: Downloads in parallel, executes after HTML parsing completes, maintains execution order
• async: Downloads in parallel, executes immediately when ready, no guaranteed order

Use defer for scripts that depend on DOM or other scripts. Use async for independent scripts like analytics.

Incorrect (blocks rendering):

export default function Document() {
  return (
    <html>
      <head>
        <script src="https://example.com/analytics.js" />
        <script src="/scripts/utils.js" />
      </head>
      <body>{/* content */}</body>
    </html>
  )
}

Correct (non-blocking):

export default function Document() {
  return (
    <html>
      <head>
        {/* Independent script - use async */}
        <script src="https://example.com/analytics.js" async />
        {/* DOM-dependent script - use defer */}
        <script src="/scripts/utils.js" defer />
      </head>
      <body>{/* content */}</body>
    </html>
  )
}

Note: In Next.js, prefer the next/script component with strategy prop instead of raw script tags:

import Script from 'next/script'

export default function Page() {
  return (
    <>
      <Script src="https://example.com/analytics.js" strategy="afterInteractive" />
      <Script src="/scripts/utils.js" strategy="beforeInteractive" />
    </>
  )
}

Reference: MDN - Script element