Throw a native SyntaxError if type is invalid

src/numeric-values.js

@@ -765,13 +765,20 @@ function reifyMathExpression(num) {
   const root = convertTokensToAST([...num.values]);
 
   // 4. Recursively transform the expression tree into objects
-  // TODO: Update when we have clarification on where simplify a calculation should be run:
-  // https://github.com/w3c/csswg-drafts/issues/9870
-  const numericValue = simplifyCalculation(transformToCSSNumericValue(root));
-  if (numericValue instanceof CSSUnitValue) {
-    return new CSSMathSum(numericValue);
+  const numericValue = transformToCSSNumericValue(root);
+  let simplifiedValue;
+  try {
+    // TODO: Update when we have clarification on where simplify a calculation should be run:
+    // https://github.com/w3c/csswg-drafts/issues/9870
+    simplifiedValue = simplifyCalculation(numericValue);
+  } catch (e) {
+    // Use insertRule to trigger native SyntaxError on TypeError
+    (new CSSStyleSheet()).insertRule('error', 0);
+  }
+  if (simplifiedValue instanceof CSSUnitValue) {
+    return new CSSMathSum(simplifiedValue);
   } else {
-    return numericValue;
+    return simplifiedValue;
   }
 }