Invalid css selector generated

[RobinMalfait]

Discussed in #14901

^{Originally posted by kdy1 November 7, 2024}
What version of Tailwind CSS are you using?

v3.4.14

What build tool (or framework if it abstracts the build tool) are you using?

Next.js + turbopack, but reproducible on playground.

What version of Node.js are you using?

Playground

What browser are you using?

Chrome

What operating system are you using?

macOS

Reproduction URL

https://play.tailwindcss.com/obgZakKyiO?file=css

Describe your issue

Generated CSS file is

p.my-4 {
  margin: 0px;
}
.\[\&_\:is\(p\)\]\:my-4 :is(p)p {
  margin: 0px;
}

where :is(p)p is invalid

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Originally opened here, but converted it too soon: #14900