Tailwind 4 and @parcel/transformer-postcss are not compatible!

[Javiani]

🐛 bug report

I'm using exactly this documentation to install tailwindcss v4 on Parcel, but it's raising an error when trying to start the application.

@parcel/transformer-postcss: Cannot read properties of undefined (reading 'input')

The error happens when try to import tailwindcss from an index.html file.

🎛 Configuration (.babelrc, package.json, cli command)

.postcssrc

{
  "plugins": {
    "@tailwindcss/postcss": {}
  }
}

.gitignore

.parcel-cache/
dist/
node_modules/

🤔 Expected Behavior

Expect to get styles applied in the HTML when application is running.

😯 Current Behavior

The application doesn't even starts, giving me the following error message:
@parcel/transformer-postcss: Cannot read properties of undefined (reading 'input')

💁 Possible Solution

🔦 Context

I just used the parcel documentation for creating a vanilla app, after that I followed tailwindcss documentation steps to install tailwind 4. I can't get the application running at all. But if I revert back to the v3 setup, it works just as expected.

💻 Code Sample

It should not work at all by following the setup provided by tailwind documentation if you're trying to use the v4.

🌍 Your Environment

Software	Version(s)
Parcel	2.14.0
Node	22.14.0
Yarn	1.22.21
Operating System	MacOs Sequoia 15.5

[devongovett]

This is a bug in Tailwind v4.1.6 and later. See #10161. They made a fix in tailwindlabs/tailwindcss#18083, which hasn't been released yet. For now, pin to 4.1.5 of @tailwindcss/postcss.