PostCSS Rewrite URL

npm install @csstools/postcss-rewrite-url --save-dev

PostCSS Rewrite URL lets you rewrite url values in CSS.

.foo {
    background: rewrite-url('foo.png');
}

@font-face {
    font-family: "Trickster";
    src:
        local("Trickster"),
        rewrite-url("trickster-COLRv1.otf") format("opentype");
}

/* becomes */

.foo {
    background: url("foo.png#modified");
}

@font-face {
    font-family: "Trickster";
    src:
        local("Trickster"),
        url("trickster-COLRv1.otf#modified") format("opentype");
}

Usage

Add PostCSS Rewrite URL to your project:

npm install postcss @csstools/postcss-rewrite-url --save-dev

Use it as a PostCSS plugin:

const postcss = require('postcss');
const postcssRewriteURL = require('@csstools/postcss-rewrite-url');

postcss([
    postcssRewriteURL(/* pluginOptions */)
]).process(YOUR_CSS /*, processOptions */);

Options

rewriter

Determine how urls are rewritten with the rewriter callback.

interface ValueToRewrite {
    url: string;
    urlModifiers: Array<string>;
}

interface RewriteContext {
    type: 'declaration-value' | 'at-rule-prelude';
    from: string | undefined;
    rootFrom: string | undefined;
    property?: string;
    atRuleName?: string;
}

type Rewriter = (value: ValueToRewrite, context: RewriteContext) => ValueToRewrite | false;

/** postcss-rewrite-url plugin options */
type pluginOptions = {
    rewriter: Rewriter;
};

postcssRewriteURL({
    rewriter: (value, context) => {
        console.log(value); // info about the `rewrite-url()` function itself (e.g. the url and url modifiers)
        console.log(context); // context surrounding the `rewrite-url()` function (i.e. where was it found?)

        if (value.url === 'ignore-me') {
            // return `false` to ignore this url and preserve `rewrite-url()` in the output
            return false;
        }

        // use url modifiers to trigger specific behavior
        if (value.urlModifiers.includes('--a-custom-modifier')) {
            return {
                url: value.url + '#other-modification',
                urlModifiers: [], // pass new or existing url modifiers to emit these in the final result
            };
        }

        return {
            url: value.url + '#modified',
        };
    },
})

Syntax

PostCSS Rewrite URL is non-standard and is not part of any official CSS Specification.

rewrite-url() function

The rewrite-url() function takes a url string and optional url modifiers and will be transformed to a standard url() function by a dev tool.

.foo {
    background: rewrite-url('foo.png');
}

rewrite-url() = rewrite-url( <string> <url-modifier>* )

Stylelint

Stylelint is able to check for unknown property values. Setting the correct configuration for this rule makes it possible to check even non-standard syntax.

"languageOptions": {
    "syntax": {
        "types": {
            "url": "| rewrite-url( <string> <url-modifier>* )"
        }
    }
},
"rules": {
    "function-no-unknown": [
        true,
        {
            "ignoreFunctions": ["rewrite-url"]
        }
    ]
}