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## With `bun dev`
When importing CSS in JavaScript-like loaders, CSS is treated special.
By default, Bun will transform a statement like this:
```js
import "../styles/global.css";
```
### When `platform` is `browser`
```js
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`
```js
//@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()`.
```ts
// 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:
```css
@import url("./hi.css");
@import url("./hello.css");
@import url("./yo.css");
```
Becomes:
```css
/* 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.
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