Attach to window, add some editable markers, and add a license file.

go-back-with-backspace/is_editable.js

 // Determine whether focus is in an editable text field.
-function isEditable(path) {
-  var target = path[0];
-
+function isEditable(target) {
   // Elements may be explicitly marked as editable.
   if (target.isContentEditable)
     return true;
 
+  // Entire documents may be editable.
+  if (target.ownerDocument.designMode.toLowerCase() === 'on')
+    return true;
+
   // Many types of input fields are editable, but not all (e.g., checkboxes).
   var nodeName = target.nodeName.toUpperCase();
   var nodeType = target.type || '';
   nodeType = nodeType.toLowerCase();
   if (nodeName === 'TEXTAREA' ||
       (nodeName === 'INPUT' && (nodeType === 'text' ||
                                 nodeType === 'password' ||
                                 nodeType === 'search' ||
                                 nodeType === 'date' ||
                                 nodeType === 'datetime' ||
                                 nodeType === 'datetime-local' ||
                                 nodeType === 'email' ||
                                 nodeType === 'month' ||
                                 nodeType === 'number' ||
                                 nodeType === 'tel' ||
                                 nodeType === 'time' ||
                                 nodeType === 'url' ||
                                 nodeType === 'week'))) {
     return true;
   }
 
   // Certain CSS styles, on elements or their parents, also indicate editable
   // fields.
-  var pathLength = path.length;
-  for (var i = 0; i < pathLength; ++i) {
-    target = path[i];
+  while (target) {
     if (target.nodeType == 1) { // Only Elements have computed styles.
-      var userModify = getComputedStyle(path[i])['-webkit-user-select'];
-      if (userModify == 'read-write' || userModify == 'write-only')
+      var userModify = getComputedStyle(target)['-webkit-user-modify'];
+      if (userModify === 'read-write' ||
+          userModify === 'write-only' ||
+          userModify === 'read-write-plaintext-only') {
         return true;
+      }
     }
+    target = target.parentNode || null;
   }
   return false;
 }