diff --git a/src/css/values/calc.rs b/src/css/values/calc.rs
index 45fc6c521542..cda07c41a6af 100644
--- a/src/css/values/calc.rs
+++ b/src/css/values/calc.rs
@@ -1665,10 +1665,6 @@ calc_protocol_forwarders!(Length {
     try_sign: forward,
     try_map: forward,
     try_op: |this, rhs, ctx, f| {
-        // Delegate to `protocol::TryOp for LengthValue` (which inlines the
-        // same-unit/px-convert dispatch) rather than the inherent `Length::try_op`
-        // — the inherent takes a 2-arg `Fn` closure, and adapting our 3-arg `f`
-        // to that without `C: Copy` would only yield `FnOnce`.
         if let (Length::Value(a), Length::Value(b)) = (this, rhs) {
             return <LengthValue as protocol::TryOp>::try_op(a, b, ctx, f).map(Length::Value);
         }
diff --git a/src/css/values/length.rs b/src/css/values/length.rs
index 62c8e3a275c8..61452164296d 100644
--- a/src/css/values/length.rs
+++ b/src/css/values/length.rs
@@ -681,20 +681,11 @@ impl protocol::TryMap for LengthValue {
 impl protocol::TryOp for LengthValue {
     #[inline]
     fn try_op<C>(&self, rhs: &Self, ctx: C, f: impl Fn(C, f32, f32) -> f32) -> Option<Self> {
-        // `LengthValue::try_op` takes a 2-arg closure (ctx folded
-        // in by caller); the `protocol::TryOp` shape passes `C` by-value with
-        // no `Copy` bound, so we can't capture `ctx` in an `Fn` closure that
-        // might be called more than once. Inline the same-unit + px-convert
-        // dispatch here so `f` is invoked exactly once on the chosen path.
         if core::mem::discriminant(self) == core::mem::discriminant(rhs) {
             let v = f(ctx, self.value(), rhs.value());
             return Some(self.map_value(|_| v));
         }
-        // Intentionally calls `self.to_px()` for BOTH operands (sic) — kept
-        // for behavioral compatibility.
-        if let (Some(a), Some(b)) = (self.to_px(), self.to_px()) {
-            return Some(LengthValue::Px(f(ctx, a, b)));
-        }
+        // A px fold can land a whole step off: `to_px` rounds, and round()/rem()/mod() jump.
         None
     }
 }
@@ -704,9 +695,7 @@ impl protocol::TryOpTo for LengthValue {
         if core::mem::discriminant(self) == core::mem::discriminant(rhs) {
             return Some(f(ctx, self.value(), rhs.value()));
         }
-        // Intentionally calls `self.to_px()` for BOTH operands (sic) — kept
-        // for behavioral compatibility.
-        if let (Some(a), Some(b)) = (self.to_px(), self.to_px()) {
+        if let (Some(a), Some(b)) = (self.to_px(), rhs.to_px()) {
             return Some(f(ctx, a, b));
         }
         None
diff --git a/test/js/bun/css/color.test.ts b/test/js/bun/css/color.test.ts
index 64477fbebbc0..b466bf952b65 100644
--- a/test/js/bun/css/color.test.ts
+++ b/test/js/bun/css/color.test.ts
@@ -574,6 +574,13 @@ describe("input forms", () => {
     expect(color("#gg0000", "hex")).toBeNull();
   });
 
+  // atan2(96px, 10px) is 84.05deg. A round() over two units is not evaluated, so atan2() has
+  // no number to use and the color cannot be converted.
+  test("a hue from atan2() of lengths in different units", () => {
+    expect(color("hsl(atan2(1in, 10px) 50% 50%)", "css")).toBe("#8cbf40");
+    expect(color("hsl(atan2(round(1in, 10px), 1px) 50% 50%)", "css")).toBeNull();
+  });
+
   test("alpha survives the object and array forms", () => {
     expect(color("#f00", "{rgba}")).toEqual({ r: 255, g: 0, b: 0, a: 1 });
     expect(color("#f00", "[rgba]")).toEqual([255, 0, 0, 255]);
diff --git a/test/js/bun/css/css.test.ts b/test/js/bun/css/css.test.ts
index 6e7b32d3b5e6..694ede631695 100644
--- a/test/js/bun/css/css.test.ts
+++ b/test/js/bun/css/css.test.ts
@@ -233,6 +233,70 @@ describe("css tests", () => {
     ); // ideally -400% - 8vh + 3ic
     minify_test(`a { top: calc(100% - 1 * 2 - 8 * 2); }`, `a{top:calc(100% - 2 - 16)}`); // ideally 100% - 18
   });
+  describe("math functions with mixed length units", () => {
+    // round(), rem(), mod() and hypot() over two lengths in different units stay as written.
+    // The second length can have no px value (1rem). If it has one, its px value is rounded
+    // (1pt is 1.3333334px), and round(), rem() and mod() can then land a whole step away.
+    // From WPT css/css-values/round-mod-rem-computed.html:
+    minify_test(`a { width: round(100px, 1rem) }`, `a{width:round(100px,1rem)}`);
+    minify_test(`a { width: mod(100px, 1rem) }`, `a{width:mod(100px,1rem)}`);
+    minify_test(`a { width: rem(100px, 1rem) }`, `a{width:rem(100px,1rem)}`);
+    // From the lightningcss tests:
+    minify_test(`a { width: round(22px, 5vw) }`, `a{width:round(22px,5vw)}`);
+    minify_test(`a { width: rem(18px, 5vw) }`, `a{width:rem(18px,5vw)}`);
+    minify_test(`a { width: mod(18px, 5vw) }`, `a{width:mod(18px,5vw)}`);
+    minify_test(`a { width: hypot(1px, 2vw) }`, `a{width:hypot(1px,2vw)}`);
+    minify_test(`a { width: hypot(1px, 2px, 3vw) }`, `a{width:hypot(1px,2px,3vw)}`);
+    // From MDN:
+    minify_test(`a { margin: mod(1000px, 29rem) }`, `a{margin:mod(1000px,29rem)}`);
+    minify_test(`a { margin: rem(1000px, 29rem) }`, `a{margin:rem(1000px,29rem)}`);
+    minify_test(`a { width: round(1in, 1em) }`, `a{width:round(1in,1em)}`);
+    minify_test(`a { width: round(1em, 1in) }`, `a{width:round(1em,1in)}`);
+    // Two absolute units.
+    minify_test(`a { width: round(1in, 10px) }`, `a{width:round(1in,10px)}`);
+    minify_test(`a { width: round(down, 70mm, 1cm) }`, `a{width:round(down,70mm,1cm)}`);
+    minify_test(`a { width: rem(1in, 10px) }`, `a{width:rem(1in,10px)}`);
+    minify_test(`a { width: mod(4px, 1pt) }`, `a{width:mod(4px,1pt)}`);
+    minify_test(`a { width: hypot(1in, 10px) }`, `a{width:hypot(1in,10px)}`);
+    minify_test(`a { width: hypot(1in, 3px, 4px) }`, `a{width:hypot(1in,3px,4px)}`);
+    minify_test(`a { border-spacing: round(1in, 10px) }`, `a{border-spacing:round(1in,10px)}`);
+    // The function also stays inside an outer function and in a media query.
+    minify_test(`a { width: calc(round(1in, 10px) + 1px) }`, `a{width:calc(round(1in,10px) + 1px)}`);
+    minify_test(`a { width: min(round(down, 70mm, 1cm), 250px) }`, `a{width:min(round(down,70mm,1cm),250px)}`);
+    minify_test(
+      `@media (min-width: round(1in, 10px)) { a { color: red } }`,
+      `@media (width>=round(1in,10px)){a{color:red}}`,
+    );
+    // One unit still folds.
+    minify_test(`a { width: round(18px, 5px) }`, `a{width:20px}`);
+    minify_test(`a { width: mod(18px, 5px) }`, `a{width:3px}`);
+    minify_test(`a { width: hypot(3in, 4in) }`, `a{width:5in}`);
+    minify_test(`a { width: round(calc(1in + 1px), 10px) }`, `a{width:100px}`);
+    // atan2() has no step, so it reads both lengths in px. (The angle is printed in whichever
+    // of deg/rad is shorter: 1.467rad is 84.0531deg.)
+    minify_test(`a { transform: rotate(atan2(1in, 0px)) }`, `a{transform:rotate(90deg)}`);
+    minify_test(`a { transform: rotate(atan2(1in, 10px)) }`, `a{transform:rotate(1.467rad)}`);
+    minify_test(`a { transform: rotate(atan2(10px, -1in)) }`, `a{transform:rotate(3.0378rad)}`);
+    minify_test(`a { rotate: atan2(1cm, 1mm) }`, `a{rotate:1.47113rad}`);
+    // A length with no px value leaves atan2() as written.
+    minify_test(`a { transform: rotate(atan2(1in, 1em)) }`, `a{transform:rotate(atan2(1in,1em))}`);
+    minify_test(`a { rotate: atan2(1px, -1vw) }`, `a{rotate:atan2(1px,-1vw)}`);
+    // An at-rule prelude cannot hold an atan2() that is left as written, so the rule is an error.
+    minify_test(
+      `@property --x { syntax: '<angle>'; inherits: false; initial-value: atan2(1in, 10px) }`,
+      `@property --x{syntax:"<angle>";inherits:false;initial-value:1.467rad}`,
+    );
+    test.each([
+      [`@media (min-width: calc(100px * tan(atan2(1in, 1em)))) { a { color: red } }`, "Invalid media query"],
+      [`@container (min-width: calc(100px * tan(atan2(1in, 1em)))) { a { color: red } }`, "Invalid media query"],
+      [
+        `@property --x { syntax: '<angle>'; inherits: false; initial-value: atan2(1in, 1em) }`,
+        "Unexpected token: atan2(",
+      ],
+    ])("ERROR: %s", (source, message) => {
+      expect(() => minify_test_with_options(source, "")).toThrow(message);
+    });
+  });
   describe("border_spacing", () => {
     minify_test(
       `