Big5 encoding quirk. Per whatwg/encoding@7f9dda7

lib/encoding.js

@@ -917,6 +917,29 @@ if (typeof module !== "undefined" && module.exports) {
     return pointer;
   }
 
+  /**
+   * @param {number} code_point The |code_point| to search for in the big5 index.
+   * @return {?number} The code point corresponding to |pointer| in |index|,
+   *     or null if |code point| is not in the big5 index.
+   */
+  function indexBig5PointerFor(code_point) {
+
+    // 1. Let index be index big5.
+    var index_ = index('big5');
+
+    // 2. If code point is U+2550, U+255E, U+2561, U+256A, U+5341, or
+    // U+5345, return the last pointer corresponding to code point in
+    // index.
+    if (code_point === 0x2550 || code_point === 0x255E ||
+        code_point === 0x2561 || code_point === 0x256A ||
+        code_point === 0x5341 || code_point === 0x5345) {
+      return index.lastIndexOf(code_point);
+    }
+
+    // 3. Return the index pointer for code point in index.
+    return indexPointerFor(code_point, index_);
+  }
+
   //
   // 7. API
   //
@@ -1864,9 +1887,8 @@ if (typeof module !== "undefined" && module.exports) {
       if (inRange(code_point, 0x0000, 0x007F))
         return code_point;
 
-      // 3. Let pointer be the index pointer for code point in index
-      // big5.
-      var pointer = indexPointerFor(code_point, index('big5'));
+      // 3. Let pointer be the index big5 pointer for code point.
+      var pointer = indexBig5PointerFor(code_point, index('big5'));
 
       // 4. If pointer is null, return error with code point.
       if (pointer === null)