next.js/packages/next/src/server/app-render/render-css-resource.tsx

import type { CssResource } from '../../build/webpack/plugins/flight-manifest-plugin'
import { encodeURIPath } from '../../shared/lib/encode-uri-path'
import type { AppRenderContext } from './app-render'
import { getAssetQueryString } from './get-asset-query-string'
import type { PreloadCallbacks } from './types'

/**
 * Abstracts the rendering of CSS files based on whether they are inlined or not.
 * For inlined CSS, renders a <style> tag with the CSS content directly embedded.
 * For external CSS files, renders a <link> tag pointing to the CSS file.
 */
export function renderCssResource(
  entryCssFiles: CssResource[],
  ctx: AppRenderContext,
  preloadCallbacks?: PreloadCallbacks
) {
  return entryCssFiles.map((entryCssFile, index) => {
    // `Precedence` is an opt-in signal for React to handle resource
    // loading and deduplication, etc. It's also used as the key to sort
    // resources so they will be injected in the correct order.
    // During HMR, it's critical to use different `precedence` values
    // for different stylesheets, so their order will be kept.
    // https://github.com/facebook/react/pull/25060
    const precedence =
      process.env.NODE_ENV === 'development'
        ? 'next_' + entryCssFile.path
        : 'next'

    // In dev, Safari and Firefox will cache the resource during HMR:
    // - https://github.com/vercel/next.js/issues/5860
    // - https://bugs.webkit.org/show_bug.cgi?id=187726
    // Because of this, we add a `?v=` query to bypass the cache during
    // development. We need to also make sure that the number is always
    // increasing.
    const fullHref = `${ctx.assetPrefix}/_next/${encodeURIPath(
      entryCssFile.path
    )}${getAssetQueryString(ctx, true)}`

    if (entryCssFile.inlined && !ctx.parsedRequestHeaders.isRSCRequest) {
      return (
        <style
          key={index}
          nonce={ctx.nonce}
          // @ts-ignore
          precedence={precedence}
          href={fullHref}
        >
          {entryCssFile.content}
        </style>
      )
    }

    preloadCallbacks?.push(() => {
      ctx.componentMod.preloadStyle(
        fullHref,
        ctx.renderOpts.crossOrigin,
        ctx.nonce
      )
    })

    return (
      <link
        key={index}
        rel="stylesheet"
        href={fullHref}
        // @ts-ignore
        precedence={precedence}
        crossOrigin={ctx.renderOpts.crossOrigin}
        nonce={ctx.nonce}
      />
    )
  })
}