Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/kind-elbakyan-fgg4tv?workspaceId=ws_NduBwxz3MLYn31fcaz2JTB

To Reproduce

Import a .css file
Start the application in dev mode
A warning is logged for each css file:

<link rel=preload> must have a valid `as` value

Current vs. Expected behavior

Stylesheets should be correctly loaded, and not trigger a browser warning.

It appears because as="stylesheet" is not a valid as value. It should be style according toMDN.

<linkrel="preload"href="/_next/static/chunks/%5Broot-of-the-server%5D__02ec3763._.css"as="stylesheet"/>

Provide environment information

Operating System:  Platform: darwin  Arch: arm64  Version: Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:29 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6000  Available memory (MB): 16384  Available CPU cores: 10Binaries:  Node: 22.20.0  npm: 10.8.2  Yarn: N/A  pnpm: 10.17.1Relevant Packages:  next: 15.6.0-canary.45 // Latest available version is detected (15.6.0-canary.45).  eslint-config-next: N/A  react: 19.2.0  react-dom: 19.2.0  typescript: 5.9.3Next.js Config:  output: N/A

Which area(s) are affected? (Select all that apply)

Not sure

Which stage(s) are affected? (Select all that apply)

next dev (local), Vercel (Deployed), next start (local), Other (Deployed)

Additional context

The issue started showing up in 15.6.0-canary.32. Previous versions do not include a preload link for stylesheets.