diff --git a/lib/index.js b/lib/index.js
index 55f56c8..6c91fd4 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -252,18 +252,23 @@ exports.DOMString = (value, options = {}) => {
 
 exports.ByteString = (value, options = {}) => {
   const x = exports.DOMString(value, options);
-  let c;
-  for (let i = 0; (c = x.codePointAt(i)) !== undefined; ++i) {
-    if (c > 255) {
-      throw makeException(TypeError, "is not a valid ByteString", options);
-    }
+
+  // Unicode regex is identical here but is ~5x slower on 16-bit strings
+  // eslint-disable-next-line require-unicode-regexp
+  if (!/[^\x00-\xff]/.test(x)) {
+    return x;
   }
 
-  return x;
+  throw makeException(TypeError, "is not a valid ByteString", options);
 };
 
 exports.USVString = (value, options = {}) => {
   const S = exports.DOMString(value, options);
+
+  if (S.toWellFormed) {
+    return S.toWellFormed();
+  }
+
   const n = S.length;
   const U = [];
   for (let i = 0; i < n; ++i) {