data:text/css imports fail in Pages Router with Turbopack

[jantimon]

Link to the code that reproduces this issue

https://github.com/jantimon/next.js/tree/fix/css-data-url-pages-router/test/e2e/css-data-url-global-pages

To Reproduce

1. Create a pages-only project (no app/ directory)
2. Import CSS via a data URL from any page:

// pages/index.tsx
import 'data:text/css,#styled{font-weight:700}'

export default function Home() {
  return <div id="styled">This text should be bold</div>
}

3. next dev --turbopack
4. Visit /

Global CSS cannot be imported from files other than your Custom <App>.
Due to the Global nature of stylesheets, and to avoid conflicts,
Please move all first-party global CSS imports to pages/_app.js.
Or convert the import to Component-Level CSS (CSS Modules).

Same import works fine in App Router (fun fact: it also works in mixed app+pages projects -> just pages-only projects are affected)

Current vs. Expected behavior

As data:text/css,... is global CSS the Pages Router restriction kicks in and blocks it (at least outside _app.tsx)

The restriction makes sense for guiding users towards CSS Modules, but nobody writes import 'data:text/css,...' by hand. It's generated by tooling (compilers, loaders) that already handles scoping. At that point the guardrail just gets in the way

Real-world impact: this blocks next-yak on Turbopack + Pages Router. In webpack mode, next-yak emits .yak.module.css via inline resource syntax (!=!), so webpack treats it as a CSS Module. Turbopack doesn't support !=!, so next-yak falls back to data:text/css;base64,..., which hits this restriction

              Webpack                           Turbopack
App Router    .yak.module.css via !=! → works   data:text/css → works
Pages Router  .yak.module.css via !=! → works   data:text/css → fails

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
Relevant Packages:
  next: 16.2.0-canary (latest canary)
  react: 19.x
  typescript: 5.x

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

CSS, Pages Router, Turbopack

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

next dev (local)

Additional context

the fix would be small:

the global CSS validation in crates/next-api/src/module_graph.rs already exempts node_modules imports. same treatment for data URL modules (path filename starts with data:) fixes this