Keep zero-length units inside all CSS math functions

fixtures/compress/units/5.css

@@ -0,0 +1,22 @@
+/*
+  issue #426 added min()/max()/clamp() to the math functions where a zero value
+  keeps its unit. The same rule applies to the other CSS math functions: inside
+  round(), mod(), rem(), hypot(), abs() etc. a unitless zero is a <number>, not a
+  zero <length>, so dropping the unit turns a valid value into an invalid one.
+*/
+
+.round {
+    width: round(0px, 3px);
+    height: round(nearest, 0px, 3px);
+}
+.mod-rem {
+    width: mod(0px, 3px);
+    height: rem(0px, 3px);
+}
+.hypot-abs {
+    width: hypot(0px, 3px);
+    height: abs(0px);
+}
+.nested {
+    width: calc(1px + round(0px, 3px));
+}

lib/replace/Dimension.js

@@ -2,9 +2,36 @@ import { packNumber } from './Number.js';
 
 const MATH_FUNCTIONS = new Set([
     'calc',
+
+    // comparison functions
     'min',
     'max',
-    'clamp'
+    'clamp',
+
+    // stepped value functions
+    'round',
+    'mod',
+    'rem',
+
+    // trigonometric functions
+    'sin',
+    'cos',
+    'tan',
+    'asin',
+    'acos',
+    'atan',
+    'atan2',
+
+    // exponential functions
+    'pow',
+    'sqrt',
+    'hypot',
+    'log',
+    'exp',
+
+    // sign-related functions
+    'abs',
+    'sign'
 ]);
 const LENGTH_UNIT = new Set([
     // absolute length units
@@ -49,7 +76,8 @@ export default function compressDimension(node, item) {
             return;
         }
 
-        // issue #222: don't remove units inside calc
+        // issue #222: don't remove units inside a math function (calc(), round(), etc.),
+        // where a unitless zero is a <number> rather than a zero <length>
         if (this.function && MATH_FUNCTIONS.has(this.function.name)) {
             return;
         }