diff --git a/substitutions/js/substitutions.js b/substitutions/js/substitutions.js
index 5b47a8e..1f9970e 100644
--- a/substitutions/js/substitutions.js
+++ b/substitutions/js/substitutions.js
@@ -63,45 +63,87 @@ chrome.runtime.sendMessage("config", function(response) {
     "TEXTAREA": 0
   };
 
+  // Set up the functions we'll need to perform the iteration.
+  var node;
+  var iter;
+
+  var filter = {
+      acceptNode: function(node) {
+          if (node.nodeType == Node.TEXT_NODE) {
+              // This is a text node that we should show to the
+              // filter.
+              return NodeFilter.FILTER_ACCEPT;
+          }
+          if (node.nodeType == Node.ELEMENT_NODE) {
+              // Fold to upper case before checking the tag name,
+              // since this may be XHTML etc.
+              if (node.tagName.toUpperCase() in ignore) {
+                  // Ignore this element, and all its children.
+                  return NodeFilter.FILTER_REJECT;
+              }
+              if (node.classList.contains("xkcdSubstitutionsExtensionSubbed")) {
+                  // We've already changed this text.  Note that some
+                  // other extension or the page's own scripts may
+                  // have made more changes to what we did, so don't
+                  // fight back and forth, constantly changing the
+                  // DOM.  Instead, just skip this subtree entirely.
+                  return NodeFilter.FILTER_REJECT;
+              }
+          }
+          // This is not a node we're interested in.  Skip this node,
+          // but process its children.
+          return NodeFilter.FILTER_SKIP;
+      }
+  };
+
   function substitute(node) {
-    "use strict";
     var replacementIdx;
     var splitIdx;
-    var parent = node.parentElement;
-    if (parent) {
-      if (parent.tagName in ignore) {
-        return;
-      }
-      var cls = parent.getAttribute("class");
-      if (cls && cls.indexOf("xkcdSubstitutionsExtensionSubbed") != -1) {
-        return;
-      }
-    }
+
+    // Before starting, make sure there's something to substitute.
+    // Otherwise, we end up doing a lot of expensive tree modification
+    // for no reason.
     if (!node.nodeValue.match(originalsRegexp)) {
       return;
     }
-    var splits = node.nodeValue.split(originalsRegexp);
+
+    // Prepare a document fragment to hold the result.
     var docFrag = document.createDocumentFragment();
+
+    // Split the string into substring, where each substring either contains
+    // something we'll substitute, or something that we won't.  We do this
+    // by using the capturing parentheses in originalsRegexp.
+    var splits = node.nodeValue.split(originalsRegexp);
     for (splitIdx = 0; splitIdx < splits.length; splitIdx++) {
       var splitString = splits[splitIdx];
       var splitStringLower = splitString.toLowerCase();
       var newNode;
       if (splitStringLower in replacementsMap) {
+        // This is something that needs to be changed.
         newNode = document.createElement("span");
         newNode.setAttribute("class", "xkcdSubstitutionsExtensionSubbed");
         newNode.setAttribute("title", splitString);
         newNode.textContent = matchCase(replacementsMap[splitStringLower],
                                         splitString);
       } else {
+        // This is a stretch between stuff that needs changing.
         newNode = document.createTextNode(splitString);
       }
       docFrag.appendChild(newNode);
     }
+
+    // Let the tree walker know that its place has changed: the old
+    // node it sent us is gone, and so we'll update its current place
+    // to refer to the last node we've processed.
+    iter.currentNode = docFrag.lastChild;
+    // Make the changes.
     node.parentNode.replaceChild(docFrag, node);
   }
 
-  var node, iter;
-  var iter = document.createNodeIterator(document.body, NodeFilter.SHOW_TEXT);
+  iter = document.createTreeWalker(document.body, 
+                                   NodeFilter.SHOW_ELEMENT |
+                                   NodeFilter.SHOW_TEXT,
+                                   filter);
   while ((node = iter.nextNode())) {
     substitute(node);
   }