---
# Don't modify this file manually. This file is auto generated from source, and you will lose your changes next time the website is built.
# Head to the `biomejs/biome` repository, and modify the source code in there.

title: noUnknownFunction
description: Learn more about noUnknownFunction
---
import { Tabs, TabItem } from '@astrojs/starlight/components';

<Tabs>
<TabItem label="CSS" icon="seti:css">
## Summary
- Rule available since: `v1.8.0`
- Diagnostic Category: [`lint/correctness/noUnknownFunction`](/reference/diagnostics#diagnostic-category)
- This rule is **recommended**, meaning it is enabled by default.
- This rule doesn't have a fix.
- The default severity of this rule is [**error**](/reference/diagnostics#error).
- Sources: 
  - Same as [`function-no-unknown`](https://github.com/stylelint/stylelint/blob/main/lib/rules/function-no-unknown/README.md)

## How to configure
```json title="biome.json"
{
	"linter": {
		"rules": {
			"correctness": {
				"noUnknownFunction": "error"
			}
		}
	}
}

```
## Description
Disallow unknown CSS value functions.

This rule ignores double-dashed custom functions, e.g. `--custom-function()`.

Data sources of known CSS value functions are:

- MDN reference on [CSS value functions](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Functions)
- MDN reference on [CSS reference](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference)
- MDN [browser compatibility data for CSS value functions](https://github.com/mdn/browser-compat-data/tree/main/css/types)

## Examples

### Invalid

```css
a { transform: unknown(1); }
```

<pre class="language-text"><code class="language-text">code-block.css:1:16 <a href="https://biomejs.dev/linter/rules/no-unknown-function">lint/correctness/noUnknownFunction</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Unexpected unknown function: </span><span style="color: Tomato;"><strong>unknown</strong></span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>a &#123; transform: unknown(1); &#125;<br />   <strong>   │ </strong>               <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>2 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Use a known function instead.</span><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">See </span><span style="color: lightgreen;"><a href="https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Functions">MDN web docs</a></span><span style="color: lightgreen;"> for more details.</span><br />  <br /></code></pre>

### Valid

```css
a { transform: scale(1); }
```

## Related links

- [Disable a rule](/linter/#disable-a-rule)
- [Configure the code fix](/linter#configure-the-code-fix)
- [Rule options](/linter/#rule-options)
- [Source Code](https://github.com/biomejs/biome/blob/main/crates/biome_css_analyze/src/lint/correctness/no_unknown_function.rs)
- [Test Cases](https://github.com/biomejs/biome/blob/main/crates/biome_css_analyze/tests/specs/correctness/noUnknownFunction)

</TabItem>
</Tabs>