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CSS @function

Proposed by web-platform-tests/interop#1357
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The @function CSS at-rule enables defining CSS custom functions. Once defined, a custom function can be called using the <dashed-function> syntax (for example, --my-function(30px, 3)) within any property value.

This feature has shipped in Chrome 139, with Safari including it in Safari Tech Preview 249.

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Strict feedback template: Please be as concrete as possible. Specific examples from your production apps or projects provide the highest-value signal to standards editors and browser engineers.
Add your specific real-world use cases, even if they seem obvious to you.
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Are you having to use another feature instead, a polyfill, or is it blocking you completely? Why are the alternatives worse?
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