Link preload must have a valid `as`

[thebuilder]

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 to MDN.

<link rel="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: 10
Binaries:
  Node: 22.20.0
  npm: 10.8.2
  Yarn: N/A
  pnpm: 10.17.1
Relevant 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: 5.9.3
Next.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.

[github-actions]

We could not detect a valid reproduction link. Make sure to follow the bug report template carefully.

Why was this issue closed?

To be able to investigate, we need access to a reproduction to identify what triggered the issue. We need a link to a public GitHub repository (template for App Router, template for Pages Router), but you can also use these templates: CodeSandbox: App Router or CodeSandbox: Pages Router.

The bug template that you filled out has a section called "Link to the code that reproduces this issue", which is where you should provide the link to the reproduction.

• If you did not provide a link or the link you provided is not valid, we will close the issue.
• If you provide a link to a private repository, we will close the issue.
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What should I do?

Depending on the reason the issue was closed, you can do the following:

• If you did not provide a link, please open a new issue with a link to a reproduction.
• If you provided a link to a private repository, please open a new issue with a link to a public repository.
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In general, assume that we should not go through a lengthy onboarding process at your company code only to be able to verify an issue.

My repository is private and cannot make it public

In most cases, a private repo will not be a sufficient minimal reproduction, as this codebase might contain a lot of unrelated parts that would make our investigation take longer. Please do not make it public. Instead, create a new repository using the templates above, adding the relevant code to reproduce the issue. Common things to look out for:

• Remove any code that is not related to the issue. (pages, API routes, components, etc.)
• Remove any dependencies that are not related to the issue.
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• Remove any environment variables that are not related to the issue.
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• If the issue is not related to a monorepo specifically, try to reproduce the issue without a complex monorepo setup

I did not open this issue, but it is relevant to me, what can I do to help?

Anyone experiencing the same issue is welcome to provide a minimal reproduction following the above steps by opening a new issue.

I think my reproduction is good enough, why aren't you looking into it quickly?

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Useful Resources

• How to Contribute to Open Source (Next.js)
• How to create a Minimal, Complete, and Verifiable example
• Reporting a Next.js bug
• Next.js Triaging issues

[icyJoseph]

Looks like the correct value should be as="style"

I'll start to triage.

[icyJoseph]

Error happens in both --turbopack and --webpack 🤔 - Hope it is not upstream related.

[icyJoseph]

I think it might have been introduced here, https://github.com/facebook/react/pull/34604/files#diff-bc667a515cbdb4a200c4460bd9ff065a60598fe8d6bdc939c922584bc2a856bdR2094 - gonna ask around

[icyJoseph]

Also re-opening issue cuz reproducing the issue is trivial