mintcss

Small, dependency-free CSS-in-JS with optional Next.js App Router integration.

Install

npm install mintcss

mintcss is an ESM package with bundled TypeScript declarations. It targets modern browsers with String.replaceAll support. Importing it does not touch the DOM; the top-level styling functions require a browser document with a head. For server rendering, use createStyleRegistry or the Next.js integration below.

Usage

import { css, globalCss, keyframes } from "mintcss";

globalCss({
    ":root": { "--accent": "#f4bd36" },
    body: { margin: 0, fontFamily: "system-ui, sans-serif" },
});

const fade = keyframes({
    from: { opacity: 0 },
    to: { opacity: 1 },
});

const button = css({
    backgroundColor: "var(--accent)",
    borderRadius: 8,
    padding: "8px 16px",
    animation: `${fade} 150ms ease-out`,
    "&:hover": { opacity: 0.8 },
    "& > svg": { width: 16 },
    "@media (width >= 40rem)": { padding: "12px 24px" },
    "@supports (display: grid)": { display: "grid" },
});

document.querySelector("button")?.classList.add(button);

In React, pass the result to className:

import { css } from "mintcss";

const button = css({ backgroundColor: "#f4bd36", borderRadius: 8 });

export function Button() {
    return <button className={button}>Hello</button>;
}

Next.js App Router

The optional mintcss/next entry point supports Next.js 14–16 with React 18.2 or 19. React and Next.js are optional peer dependencies; the core entry point does not import either framework.

Wrap the app in MintStyleRegistry in your root layout. The layout can remain a Server Component:

// app/layout.tsx
import type { ReactNode } from "react";
import { MintStyleRegistry } from "mintcss/next";

export default function RootLayout({ children }: { children: ReactNode }) {
    return (
        <html lang="en">
            <body>
                <MintStyleRegistry>{children}</MintStyleRegistry>
            </body>
        </html>
    );
}

In Client Components, obtain the styling functions from useMintcss and call them during rendering:

// app/button.tsx
"use client";

import { useMintcss } from "mintcss/next";

export function Button() {
    const { css, globalCss, keyframes } = useMintcss();
    globalCss({ body: { margin: 0 } });
    const fade = keyframes({ from: { opacity: 0 }, to: { opacity: 1 } });
    const button = css({
        backgroundColor: "#f4bd36",
        borderRadius: 8,
        animation: `${fade} 150ms ease-out`,
        "&:hover": { opacity: 0.8 },
    });
    return <button type="button" className={button}>Hello</button>;
}

The registry collects CSS separately for each server render and uses Next.js's useServerInsertedHTML integration to emit new rules as content streams. The browser reuses those rules during hydration and inserts new rules for subsequent renders and navigation. The same styles produce the same identifiers on the server and client.

Use the hook inside Client Components under the registry. Module-level calls to the browser-only css, globalCss, or keyframes exports still require a DOM, even in files with "use client", because Next.js also renders Client Components on the server. Style objects can be declared at module scope, then passed to the hook's functions during rendering. Server Components cannot use useMintcss. The automatic integration targets the App Router; Pages Router applications need their own SSR collection using the API below.

For a nonce-based CSP, pass the request's nonce as <MintStyleRegistry nonce={nonce}>. It is applied to server-generated style tags and new browser style tags. Configure the corresponding CSP in your app.

Server rendering

createStyleRegistry({ nonce? }) provides css, globalCss, and keyframes without requiring a DOM. Create a separate registry for every server render and use its functions throughout that render; do not share a mutable registry between requests.

import { createStyleRegistry } from "mintcss";

const registry = createStyleRegistry();
const button = registry.css({ color: "red" });
const styles = registry.getStyles();
// styles: { ids: string[], css: string }
// Render a <style data-mintcss={styles.ids.join(" ")}> containing styles.css
// in the document head alongside the HTML using button as its class name.

getStyles() returns all collected rules in insertion order. flush() returns only the rules collected since the previous flush, retaining deduplication for later streaming chunks. Include data-mintcss with the emitted identifiers so browser calls can reuse the SSR styles. In the browser, registry styling calls also insert rules into the document head.

API

• css(styles: CSSObject): string inserts rules and returns a deterministic mint-… class name. Object key order does not change that name.
• keyframes(frames: Record<string, CSSDeclarations>): string inserts an animation and returns its mint-kf-… name. Steps are from, to, or integer percentages from 0% through 100%, emitted in numeric order.
• globalCss(rules: Record<string, CSSObject>): void inserts global selector rules. Top-level @media and @supports blocks contain selector objects.
• Types CSSObject, CSSDeclarations, and CSSValue are exported. Properties accept strings, numbers, and undefined; nested objects represent selectors or supported conditional rules. These types do not validate property names against a complete CSS specification.

Properties use camelCase (backgroundColor, WebkitAppearance, msOverflowStyle) except CSS custom properties (--accent). Nonzero numeric values receive px unless they are custom properties or supported unitless properties, such as opacity, lineHeight, fontWeight, flexGrow, or zIndex. Zero stays 0. Use strings for other units and numeric values that must remain unitless. undefined declarations are omitted.

Nested selectors must include &, which is replaced by the parent selector. Only @media and @supports conditional rules are supported; use keyframes for animations. Declarations and nested keys are emitted in sorted order, so avoid mixing shorthand and longhand declarations that depend on insertion order.

Rules are appended to a shared <style data-mintcss> element. Its DOM registry avoids duplicate insertion, including across module reloads. Global rules keep selector insertion order so their cascade is preserved. Generated identifiers are implementation details and should not be persisted or hard-coded.

Limits

There is no build-time extraction or automatic vendor prefixing. Styles accumulate for the lifetime of the document; prefer reusing stable style objects and CSS variables for continuously changing values. Strict CSP deployments must permit the generated inline stylesheet, for example with the registry nonce option.

The serializer rejects malformed properties and rules, non-finite numeric values, and certain injection-like strings. It is not an untrusted-input sanitizer: pass application-authored styles and validate external data before using it, particularly URLs. It does not enforce an external URL policy.