Preprocess a standalone surrogate to \uFFFD

- Also optimize surrogate pair handling using native String.codePointAt().
- ref: https://www.w3.org/TR/css-syntax-3/#input-preprocessing

parse-css.js

@@ -67,17 +67,14 @@ function preprocess(str) {
   // following the preprocessing cleanup rules.
   var codepoints = [];
   for(var i = 0; i < str.length; i++) {
-    var code = str.charCodeAt(i);
-    if(code == 0xd && str.charCodeAt(i+1) == 0xa) {
+    var code = str.codePointAt(i);
+    if (code == 0xd && str.charCodeAt(i+1) == 0xa) {
       code = 0xa; i++;
-    }
-    if(code == 0xd || code == 0xc) code = 0xa;
-    if(code == 0x0) code = 0xfffd;
-    if(between(code, 0xd800, 0xdbff) && between(str.charCodeAt(i+1), 0xdc00, 0xdfff)) {
-      // Decode a surrogate pair into an astral codepoint.
-      var lead = code - 0xd800;
-      var trail = str.charCodeAt(i+1) - 0xdc00;
-      code = Math.pow(2, 16) + lead * Math.pow(2, 10) + trail;
+    } else if (code == 0xd || code == 0xc) {
+      code = 0xa;
+    } else if (code == 0x0 || between(code, 0xd800, 0xdfff)) {
+      code = 0xfffd;
+    } else if (code > 0xffff) {
       i++;
     }
     codepoints.push(code);

tests.js

@@ -368,6 +368,36 @@ var TESTS = [
         }
       ]
     }
+  }, {
+    parser: '',
+    css: `\u{20000},\u{0},\uD800,\uDFFF`,
+    expected: [
+      {
+        "type": "IDENT",
+        "value": "\u{20000}"
+      },
+      {
+        "type": "COMMA",
+      },
+      {
+        "type": "IDENT",
+        "value": "\uFFFD"
+      },
+      {
+        "type": "COMMA",
+      },
+      {
+        "type": "IDENT",
+        "value": "\uFFFD"
+      },
+      {
+        "type": "COMMA",
+      },
+      {
+        "type": "IDENT",
+        "value": "\uFFFD"
+      },
+    ]
   }
 ];