removeUnusedAtRules removes @font-face for fonts used in CSS variable declarations

[alecglassford]

Environment

• clean-css version - 5.3.1
• node.js version: 16.16.0
• operating system: N/A

Configuration options

new CleanCSS({ level: { 2: { removeUnusedAtRules: true } } })

Input CSS

@font-face {
    font-family: test;
    src: url(https://test);
}

:root {
    --serif-stack: test;
}

p {
    font-family: var(--serif-stack);
}

Actual output CSS

:root{--serif-stack:test}p{font-family:var(--serif-stack)}

Expected output CSS

@font-face{font-family:test;src:url(https://test)}:root{--serif-stack:test}p{font-family:var(--serif-stack)}

---

In this example, it seems that because the test font is only used in a CSS variable declaration, clean-css mistakenly considers it "unused."