bun.sh/docs/dev/css.md

With bun dev

When importing CSS in JavaScript-like loaders, CSS is treated special.

By default, Bun will transform a statement like this:

import "../styles/global.css";

When platform is browser

globalThis.document?.dispatchEvent(
  new CustomEvent("onimportcss", {
    detail: "http://localhost:3000/styles/globals.css",
  }),
);

An event handler for turning that into a <link> is automatically registered when HMR is enabled. That event handler can be turned off either in a framework’s package.json or by setting globalThis["Bun_disableCSSImports"] = true; in client-side code. Additionally, you can get a list of every .css file imported this way via globalThis["__BUN"].allImportedStyles.

When platform is bun

//@import url("http://localhost:3000/styles/globals.css");

Additionally, Bun exposes an API for SSR/SSG that returns a flat list of URLs to css files imported. That function is Bun.getImportedStyles().

// This specifically is for "framework" in package.json when loaded via `bun dev`
// This API needs to be changed somewhat to work more generally with Bun.js
// Initially, you could only use Bun.js through `bun dev`
// and this API was created at that time
addEventListener("fetch", async (event: FetchEvent) => {
  let route = Bun.match(event);
  const App = await import("pages/_app");

  // This returns all .css files that were imported in the line above.
  // It’s recursive, so any file that imports a CSS file will be included.
  const appStylesheets = bun.getImportedStyles();

  // ...rest of code
});

This is useful for preventing flash of unstyled content.

With bun bun

Bun bundles .css files imported via @import into a single file. It doesn’t autoprefix or minify CSS today. Multiple .css files imported in one JavaScript file will not be bundled into one file. You’ll have to import those from a .css file.

This input:

@import url("./hi.css");
@import url("./hello.css");
@import url("./yo.css");

Becomes:

/* hi.css */
/* ...contents of hi.css */
/* hello.css */
/* ...contents of hello.css */
/* yo.css */
/* ...contents of yo.css */

CSS runtime

To support hot CSS reloading, Bun inserts @supports annotations into CSS that tag which files a stylesheet is composed of. Browsers ignore this, so it doesn’t impact styles.

By default, Bun’s runtime code automatically listens to onimportcss and will insert the event.detail into a <link rel="stylesheet" href={${event.detail}}> if there is no existing link tag with that stylesheet. That’s how Bun’s equivalent of style-loader works.