diff --git a/src/builtin/color/hsl.rs b/src/builtin/color/hsl.rs
index ab51c8f..d3ced94 100644
--- a/src/builtin/color/hsl.rs
+++ b/src/builtin/color/hsl.rs
@@ -23,7 +23,12 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             | Value::Dimension(n, Unit::Percent) => n / Number::from(100),
             _ => todo!("expected either unitless or % number for alpha"),
         };
-        Some(Value::Color(Color::from_hsla(hue, saturation, luminance, Number::from(1))))
+        let alpha = match arg!(args, 3, "alpha"=Value::Dimension(Number::from(1), Unit::None)) {
+            Value::Dimension(n, Unit::None) => n,
+            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
+            _ => todo!("non-number alpha given to builtin function `rgb()`")
+        };
+        Some(Value::Color(Color::from_hsla(hue, saturation, luminance, alpha)))
     });
     decl!(f "hsla", |args, _| {
         let hue = match arg!(args, 0, "hue").eval() {
diff --git a/tests/color.rs b/tests/color.rs
index ea49f6c..135815e 100644
--- a/tests/color.rs
+++ b/tests/color.rs
@@ -128,6 +128,11 @@ test!(
     "a {\n  color: hsl($hue: 193, $saturation: 67%, $luminance: 99);\n}\n",
     "a {\n  color: #fbfdfe;\n}\n"
 );
+test!(
+    hsl_four_args,
+    "a {\n  color: hsl(0, 0, 0, 0.456);\n}\n",
+    "a {\n  color: rgba(0, 0, 0, 0.456);\n}\n"
+);
 test!(
     hsl_negative_hue,
     "a {\n  color: hsl(-60deg, 100%, 50%);\n}\n",