--- a/devtools/client/sourceeditor/editor.js
+++ b/devtools/client/sourceeditor/editor.js
@@ -252,21 +252,23 @@ Editor.prototype = {
    *
    * This method is asynchronous and returns a promise.
    */
   appendTo: function (el, env) {
     let def = promise.defer();
     let cm = editors.get(this);
 
     if (!env) {
-      env = el.ownerDocument.createElementNS(XUL_NS, "iframe");
+      env = el.ownerDocument.createElementNS(el.namespaceURI, "iframe");
+
+      if (el.namespaceURI === XUL_NS) {
+        env.flex = 1;
+      }
     }
 
-    env.flex = 1;
-
     if (cm) {
       throw new Error("You can append an editor only once.");
     }
 
     let onLoad = () => {
       let win = env.contentWindow.wrappedJSObject;
 
       if (!this.config.themeSwitching) {