lexer: don't treat default as valid value for <custom-ident>

lib/lexer/generic.js

@@ -166,18 +166,25 @@ function expression(node) {
 }
 
 // https://developer.mozilla.org/en-US/docs/Web/CSS/custom-ident
+// https://drafts.csswg.org/css-values-4/#identifier-value
 function customIdent(node) {
     if (node.data.type !== 'Identifier') {
         return false;
     }
 
     var name = node.data.name.toLowerCase();
 
-    // can't be a global CSS value
+    // § 3.2. Author-defined Identifiers: the <custom-ident> type
+    // The CSS-wide keywords are not valid <custom-ident>s
     if (name === 'unset' || name === 'initial' || name === 'inherit') {
         return false;
     }
 
+    // The default keyword is reserved and is also not a valid <custom-ident>
+    if (name === 'default') {
+        return false;
+    }
+
     // TODO: ignore property specific keywords (as described https://developer.mozilla.org/en-US/docs/Web/CSS/custom-ident)
 
     return true;