From e027cd112a83af3f1d28589468a23fdf22dbb66d Mon Sep 17 00:00:00 2001
From: Bart Waardenburg <bart@waardenburg.dev>
Date: Thu, 22 Oct 2026 23:19:03 +0200
Subject: [PATCH] color: Normalize a non-finite hue to 0 degrees.

A hue that overflows f32, such as hsl(1e40 50% 50%), made normalize_hue return NaN. The NaN hue failed the debug assertion in hsl_to_rgb and gave wrong channels in a release build.

CSS Color 4 says that an infinite hue normalizes to 0 degrees, and CSS Values 4 censors NaN to 0.
---
 color/lib.rs   |  7 +++++++
 color/tests.rs | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+)

diff --git a/color/lib.rs b/color/lib.rs
index 4d5890e0..abc3e1a4 100644
--- a/color/lib.rs
+++ b/color/lib.rs
@@ -489,6 +489,13 @@ impl ToCss for ModernComponent<'_> {
 
 // Guaratees hue in [0..360)
 fn normalize_hue(hue: f32) -> f32 {
+    // An infinite hue normalizes to 0 degrees:
+    // <https://drafts.csswg.org/css-color-4/#hue-syntax>
+    // NaN is censored to 0:
+    // <https://drafts.csswg.org/css-values-4/#calc-ieee>
+    if !hue.is_finite() {
+        return 0.0;
+    }
     // <https://drafts.csswg.org/css-values/#angles>
     // Subtract an integer before rounding, to avoid some rounding errors:
     hue - 360.0 * (hue / 360.0).floor()
diff --git a/color/tests.rs b/color/tests.rs
index fbbb553d..87a53ce0 100644
--- a/color/tests.rs
+++ b/color/tests.rs
@@ -394,3 +394,49 @@ fn serialize_modern_components() {
         "calc(NaN)".to_string()
     );
 }
+
+fn parse_and_serialize(css: &str) -> String {
+    Color::parse(&mut Parser::new(css)).unwrap().to_css_string()
+}
+
+#[test]
+fn normalize_non_finite_hue() {
+    // <https://drafts.csswg.org/css-color-4/#hue-syntax>: an infinite hue
+    // normalizes to 0 degrees. 1e40 does not fit in an f32, so it parses
+    // as an infinity.
+    for (input, expected) in [
+        ("hsl(1e40 50% 50%)", "hsl(0 50% 50%)"),
+        ("hsl(-1e40 50% 50%)", "hsl(0 50% 50%)"),
+        ("hsl(1e40deg 50% 50%)", "hsl(0 50% 50%)"),
+        ("hsl(1e38turn 50% 50%)", "hsl(0 50% 50%)"),
+        ("hwb(1e40 0% 0%)", "hwb(0 0% 0%)"),
+        ("hwb(-1e40 0% 0%)", "hwb(0 0% 0%)"),
+        ("lch(50% 30 1e40)", "lch(50% 30 0)"),
+        ("oklch(50% 0.1 -1e40)", "oklch(50% 0.1 0)"),
+    ] {
+        assert_eq!(
+            parse_and_serialize(input),
+            parse_and_serialize(expected),
+            "{input}"
+        );
+    }
+    assert_eq!(parse_and_serialize("hwb(1e40 0% 0%)"), "rgb(255, 0, 0)");
+    assert_eq!(parse_and_serialize("lch(50% 30 1e40)"), "lch(50 30 0)");
+}
+
+#[test]
+fn normalize_finite_hue() {
+    for (input, expected) in [
+        ("hsl(480 50% 50%)", "hsl(120 50% 50%)"),
+        ("hsl(-240 50% 50%)", "hsl(120 50% 50%)"),
+        ("hsl(360 50% 50%)", "hsl(0 50% 50%)"),
+        ("hwb(-540 10% 10%)", "hwb(180 10% 10%)"),
+        ("lch(50% 30 400)", "lch(50% 30 40)"),
+    ] {
+        assert_eq!(
+            parse_and_serialize(input),
+            parse_and_serialize(expected),
+            "{input}"
+        );
+    }
+}