support special fn clamp

src/parse/value/parse.rs

@@ -278,6 +278,9 @@ impl<'a, 'b> Parser<'a, 'b> {
                                 .to_css_string(self.options.is_compressed())?,
                         ),
                     },
+                    "clamp" if lower == "clamp" => {
+                        self.parse_calc_args(&mut s)?;
+                    }
                     _ => s.push_str(
                         &self
                             .parse_call_args()?

src/value/css_function.rs

@@ -4,4 +4,5 @@ pub(crate) fn is_special_function(s: &str) -> bool {
         || s.starts_with("env(")
         || s.starts_with("min(")
         || s.starts_with("max(")
+        || s.starts_with("clamp(")
 }

tests/special-functions.rs

@@ -234,3 +234,23 @@ error!(
     progid_nothing_after,
     "a { color: progid:", "Error: expected \"(\"."
 );
+test!(
+    clamp_empty_args,
+    "a {\n  color: clamp();\n}\n",
+    "a {\n  color: clamp();\n}\n"
+);
+test!(
+    clamp_parens_in_args,
+    "a {\n  color: clamp((()));\n}\n",
+    "a {\n  color: clamp((()));\n}\n"
+);
+test!(
+    clamp_single_arg,
+    "a {\n  color: clamp(1);\n}\n",
+    "a {\n  color: clamp(1);\n}\n"
+);
+test!(
+    clamp_many_args,
+    "a {\n  color: clamp(1, 2, 3);\n}\n",
+    "a {\n  color: clamp(1, 2, 3);\n}\n"
+);