What version of Bun is running?

1.3.10

What platform is your computer?

Linux 6.12.58+ x86_64

What steps can reproduce the bug?

1. Create a project using Bun.serve() with an HTML import:

// server.ts
import index from "./index.html";
Bun.serve({
  development: true,
  routes: { "/": index },
  fetch(req) { /* ... */ },
});

2. In index.html, reference a script that imports @fontsource-variable/roboto:

<script type="module" src="./app.tsx"></script>

3. In app.tsx:

import "@fontsource-variable/roboto";

4. The fontsource package's index.css contains @font-face rules with unicode-range values like:

@font-face {
  font-family: "Roboto Variable";
  src: url(./files/roboto-latin-wght-normal.woff2) format("woff2-variations");
  unicode-range: U+0000-00FF,U+0131,U+0152-0153;
}

5. Load the page and inspect the CSS output served by Bun.

What is the expected behavior?

The unicode-range values should be preserved as-is:

unicode-range: U+0000-00FF,U+0131,U+0152-0153;

What do you see instead?

Bun's CSS processor strips the + from U+ prefixes and sometimes inserts spaces, producing invalid unicode-range values:

unicode-range: U0000-00FF, U0131, U0152-0153;

Some ranges also get mangled with spaces: U + A640-A69F instead of U+A640-A69F.

Because U0000-00FF is not valid CSS (the spec requires U+), browsers silently ignore the entire @font-face rule, causing fonts to not load at all.

Additional information

This affects any CSS with unicode-range processed by Bun's dev bundler (not just fontsource). The corruption occurs in Bun's built-in CSS minification/processing pipeline.

Workaround: Process font CSS via a custom [serve.static].plugins loader that reads the raw CSS and injects it via a <style> tag, bypassing Bun's CSS processing.