diff --git a/lib/dom-parser.js b/lib/dom-parser.js
index 5b07c1fce..023b6701e 100644
--- a/lib/dom-parser.js
+++ b/lib/dom-parser.js
@@ -1,3 +1,5 @@
+'use strict'
+
 var conventions = require("./conventions");
 var dom = require('./dom')
 var entities = require('./entities');
diff --git a/lib/index.js b/lib/index.js
index df827f6f8..85be0abc9 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -1,3 +1,5 @@
+'use strict'
+
 var dom = require('./dom')
 exports.DOMImplementation = dom.DOMImplementation
 exports.XMLSerializer = dom.XMLSerializer
diff --git a/lib/sax.js b/lib/sax.js
index 9ab6c121c..06aeba540 100644
--- a/lib/sax.js
+++ b/lib/sax.js
@@ -1,3 +1,5 @@
+'use strict'
+
 var conventions = require("./conventions");
 var NAMESPACE = conventions.NAMESPACE;
 var MIME_TYPE = conventions.MIME_TYPE;