Next.jsCSSPerformanceTurbopackSCSS

Hvordan jeg eliminerede 94 render-blokerende CSS-filer i Next.js 16 med en dårligt dokumenteret Turbopack-funktion

Efter dage med at prøve enhver tilgang — fra experimental.inlineCss til MutationObserver-hacks — opdagede jeg Turbopack Import Attributes, der løser problemet med render-blokerende CSS i Next.js App Router.

Udgivet 8. april 202612 min læsning

Problemet

Vores app (Promova) bruger Next.js 16 med en Landing Builder — et CMS-drevet system, der samler marketingsider fra ~90 forskellige sektionskomponenter. Arkitekturen bruger en sectionRegistry.tsx, der mapper sektionsnavne til next/dynamic()-kald:

sectionRegistry.tsx

// sectionRegistry.tsx — imports all 90 sections
export const sectionRegistry = {
  HeroSection: dynamic(() => import('./HeroSection/HeroSection')),
  FAQSection: dynamic(() => import('./FAQSection/FAQSection')),
  ReviewsSection: dynamic(() => import('./ReviewsSection/ReviewsSection')),
  // ... 87 more
}

En enkelt landingsside renderer kun 5-8 sektioner. Men Lighthouse viste:

Eliminate render-blocking resources
  94 CSS resources (~330 KB)
  Potential savings: 5,440 ms

Hvorfor? Turbopack ser alle 90 import()-stier som tilgængelige og genererer <link rel="stylesheet"> for hvert SCSS-modul. Selv sektioner, der aldrig renderes på siden, får deres CSS injiceret i <head>. Dette er en bekræftet, forventet adfærd i Next.js App Router. Ingen rettelse er planlagt.

Alt, hvad jeg prøvede (og hvorfor det fejlede)

Jeg brugte dage på at gennemgå enhver tilgang, jeg kunne finde. Her er den fulde liste:

Approach	Why it doesn't work
Split `sectionRegistry` into per-section files	CSS still loaded — #61066
`experimental.inlineCss: true`	Inlines CSS on every SSR request — crashed our CMS under load
`experimental.optimizeCss` (Critters)	Incompatible with streaming in App Router
CSS-in-JS rewrite	Rewriting 90 sections from SCSS to CSS-in-JS is not realistic
`media="print"` hack	Doesn't work with CSS Modules in Turbopack — `<link>` tags are managed by the framework
Switch back to Webpack	Has `experiments.css` options, but we're committed to Turbopack
Turbopack plugins	Don't exist — no plugin API
Turbopack CSS loaders	Not supported — only JS output
SWC plugins for CSS	SWC only processes JavaScript
Client Component wrapper for `dynamic()`	Registry is a global constant — bundler sees all dependencies
Next.js middleware HTML rewrite	Middleware can't modify response body
Suspense + streaming with async import	React Float always pulls CSS into initial `<head>`
Suspense + 3s delay	Content streams later, but CSS is in the initial `<head>` chunk
`experimental.cssChunking`	Already default. Merges chunks but doesn't remove irrelevant CSS
Post-build Beasties/Critters	Only for static export, not with ISR
npm libraries (next-critical, fg-loadcss)	Pages Router only or abandoned (6+ years)
`inlineCss` per-route exclusion	Not supported — all-or-nothing global flag
Turbopack `as: 'raw'` for SCSS	Returns `undefined` instead of text
Turbopack rule for `*.inline.module.scss`	Turbopack intercepts `.module.scss` before custom rules
Turbopack rule for `*.inline.scss`	Turbopack intercepts `.scss` before custom rules
MutationObserver `<script>`	Tested in dev, risky — CSS still loads as separate files, possible FOUC

inlineCss-fælden

Next.js har et experimental.inlineCss-flag, der erstatter alle <link rel="stylesheet"> med inline <style>-tags. Lyder perfekt, ikke?

Problemet: det er alt eller intet. Du kan ikke aktivere det per rute. Hvis du har SSR-sider (force-dynamic), genopbygger hver anmodning al CSS inline. Vi prøvede — vores headless CMS kunne ikke klare belastningen.

Opdagelsen: Turbopack Import Attributes

Ved at grave i Next.js 16.2 release notes fandt jeg en dårligt dokumenteret funktion: Turbopack Import Attributes. Den lader dig tilsidesætte den indbyggede bundler-pipeline for en specifik import med TC39 with {}-syntaks:

import { cssText, styles } from './hero.module.scss' with {
  turbopackLoader: '@promova/scss-to-inline-loader',
  turbopackAs: '*.js'
}

Dette fortæller Turbopack: "Behandl ikke denne import som et stilark. Kør den gennem min brugerdefinerede loader og behandl outputtet som JavaScript."

Dette er den afgørende indsigt. I stedet for at Turbopack genererer et <link rel="stylesheet">, der blokerer rendering, kompilerer vores loader SCSS og eksporterer det som en JS-streng. Resultat: kun CSS for sektioner, der faktisk renderes, ender i sidens HTML.

Løsningen

1. Brugerdefineret Turbopack Loader

Et ~70-linjers Node.js-script som yarn workspace-pakke (@promova/scss-to-inline-loader):

@promova/scss-to-inline-loader/index.js

const { createHash } = require('node:crypto')
const sass = require('sass')
const postcss = require('postcss')
const postcssModules = require('postcss-modules')
const path = require('path')

const SHARED_STYLES = path.resolve(__dirname, '../shared/styles')

// Same prependData as sassOptions.prependData in next.config.ts
const PREPEND_DATA = `
@use "sass:math" as math;
@use "sass:list" as list;
@use "${SHARED_STYLES}/_variables.scss" as *;
@use "${SHARED_STYLES}/_breakpoints.scss" as *;
@use "${SHARED_STYLES}/_mixins.scss" as *;
`

module.exports = function scssToInlineLoader(source) {
  const callback = this.async()
  processScss(source, this.resourcePath)
    .then((js) => callback(null, js))
    .catch((err) => callback(err))
}

async function processScss(source, resourcePath) {
  // 1. Compile SCSS → CSS (with global variables, mixins, breakpoints)
  const sassResult = sass.compileString(PREPEND_DATA + '\n' + source, {
    loadPaths: [path.dirname(resourcePath), SHARED_STYLES],
    style: 'compressed',
    sourceMap: false,
    url: new URL('file://' + resourcePath),
  })

  // 2. Scope class names with postcss-modules (CSS Modules compatible)
  let classNames = {}
  const result = await postcss([
    postcssModules({
      generateScopedName(name, filename) {
        const file = path.basename(filename, path.extname(filename))
          .replace('.module', '')
        const hash = createHash('md5')
          .update(filename + name)
          .digest('base64url')
          .slice(0, 6)
        return `${file}__${name}__${hash}`
      },
      getJSON(_, json) { classNames = json },
    }),
  ]).process(sassResult.css, { from: resourcePath })

  // 3. Export as JS module — same interface as CSS Modules
  return [
    `export const cssText = ${JSON.stringify(result.css)};`,
    `export const styles = ${JSON.stringify(classNames)};`,
    `export default styles;`,
  ].join('\n')
}

Hvad den gør: styles — det samme scopede klassenavnkort som standard CSS Modules. cssText — kompileret CSS som streng.

2. InlineStyle-komponent

Bruger React 19s indbyggede <style href precedence>-API til automatisk deduplicering:

InlineStyle.tsx

export function InlineStyle({ css, id }: { css: string; id: string }) {
  return (
    <style href={id} precedence="default">
      {css}
    </style>
  )
}

React 19 garanterer: samme href → kun ét <style> i DOM.

3. Migrering per komponent (~6 linjer per sektion)

// BEFORE: Turbopack → <link rel="stylesheet"> (render-blocking)
import styles from './hero_section.module.scss'

HeroSection.tsx

// AFTER: Custom loader → JS module → inline <style> (non-blocking)
import { cssText, styles } from './hero_section.module.scss' with {
  turbopackLoader: '@promova/scss-to-inline-loader',
  turbopackAs: '*.js'
}
import { InlineStyle } from '@promova/scss-to-inline-loader/InlineStyle'

export function HeroSection() {
  return (
    <section className={styles.hero}>
      <InlineStyle css={cssText} id="hero-section" />
      {/* ... component JSX, className usage is identical */}
    </section>
  )
}

.module.scss-filerne forbliver nøjagtig de samme. Ingen CSS-omskrivning.

Hvorfor dette er bedre end inlineCss: true

Her er den kritiske forskel:

	<code>inlineCss: true</code>	Import Attributes + Loader
What gets inlined	ALL CSS on the page (94 files)	Only CSS for rendered sections (5-8 files)
SSR overhead	CSS rebuilt on every request	CSS compiled at build time
Per-route control	No (all-or-nothing)	Yes (per-import)
SSR pages safety	Risky (overloads server)	Safe (component-level)

Med inlineCss: true får en side stadig ALLE 94 stilark inline. Med vores tilgang ender kun CSS, der faktisk renderes, i HTML'en.

Turbopack-fælden: ingen globale regler for .module.scss

En fælde jeg faldt i: du tror måske, at du kan tilføje en Turbopack-regel i next.config.ts for at anvende loaderen globalt:

next.config.ts

// ❌ THIS CAUSES A FATAL PANIC
turbopack: {
  rules: {
    '**/components/**/*.module.scss': {
      loaders: ['@promova/scss-to-inline-loader'],
      as: '*.js'
    }
  }
}

Gør ikke dette. Turbopacks indbyggede CSS-modul-pipeline opfanger .module.scss-filer før brugerdefinerede regler anvendes, hvilket forårsager:

FATAL PANIC: inner asset should be CSS processable

with {}-attributter fungerer, fordi de instruerer Turbopack på importstedet om at omgå CSS-modul-pipelinen fuldstændigt.

Resultater

127 sektionskomponenter migreret i Landing Builder. Produktionsbuild verificeret.

Metric	Before	After
Render-blocking CSS files	94	0 (for landing sections)
CSS in HTML per page	~330 KB (all sections)	~20-40 KB (rendered sections only)
CSS delivery	`<link>` (network request, blocking)	`<style>` (inline, non-blocking)
Sass features	Full	Full (variables, mixins, nesting, @use)
CSS Modules scoping	Built-in	postcss-modules (compatible)
Code change per section	—	~6 lines (import + InlineStyle)

Begrænsninger

with {} per import er ordrig — hver import kræver 3 ekstra linjer.
Kun Turbopack — with {}-attributter understøttes ikke af Webpack.
Klassenavnhashing — vores loader bruger en anden hashingsalgoritme end Turbopacks indbyggede.
HTML-størrelse stiger — CSS er inline i HTML i stedet for cachede separate filer.

Hvornår du skal bruge dette

Denne teknik er mest effektiv, når:

Du har et registry/barrel-mønster — én fil importerer mange komponenter, men kun få renderes per side
Du bruger Turbopack — Import Attributes er Turbopack-specifikke
Du vil have kontrol per komponent — ikke et alt-eller-intet-flag
Din SCSS er kompleks — variabler, mixins, breakpoints, nesting — alt understøttet
Du ikke kan bruge experimental.inlineCss — fordi du har SSR-sider eller ønsker granulær kontrol

Hvis du er påvirket af render-blokerende CSS i Next.js App Router — du er ikke alene:

Kerneproblemet

#62485 — Render blocking CSS (maintainers: "expected behavior")
#61066 — Dynamic imports from Server Components are not code-split
#54935 — Server-side dynamic imports don't split client modules
#61574 — JS/CSS code splitting doesn't work as documented
#57634 — Add support for critical CSS inlining with App Router
#50300 — next/dynamic on server component does not build CSS modules

inlineCss-funktionen & problemer

PR #72195 — experimental: css inlining (the implementation)
PR #73182 — Don't inline CSS in RSC payload for client navigation
#75648 — WhatsApp preview broken with inlineCss + Tailwind
#83612 — Turbopack wrong font URL with inline CSS

Fællesskabet søger løsninger

Discussion #82894 — How to prevent render-blocking with CSS Modules? (2025)
Discussion #70526 — Ideas for reducing render-blocking CSS
Discussion #59814 — Render blocking styles with Tailwind
Discussion #49691 — How to deal with render-blocking modular CSS
Discussion #59989 — Critical CSS inlining with App Router
Discussion #80486 — Is optimizeCss still in use?
Discussion #85465 — Turbopack plugin API (doesn't exist)

Turbopack CSS-fejl

#68412 — Incorrect CSS modules load order with external components
#76464 — Turbopack sometimes strips SCSS imports
#82497 — SCSS module not loading when not-found.tsx is present
#88544 — Exporting Sass variables from CSS modules doesn't work with Turbopack

Bygget hos Promova — en sproglæringsplatform, der betjener millioner af brugere.

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