diff --git a/src/ext/response-targets.js b/src/ext/response-targets.js
index 3914ba88..dd6fd418 100644
--- a/src/ext/response-targets.js
+++ b/src/ext/response-targets.js
@@ -5,6 +5,11 @@
 
     var attrPrefix = 'hx-target-';
 
+    // IE11 doesn't support string.startsWith
+    function startsWith(str, prefix) {
+        return str.substring(0, prefix.length) === prefix
+    }
+
     /**
      * @param {HTMLElement} elt
      * @param {number} respCode
@@ -38,7 +43,7 @@
             '***',
             'xxx',
         ];
-        if (respCode.startsWith('4') || respCode.startsWith('5')) {
+        if (startsWith(respCode, '4') || startsWith(respCode, '5')) {
             attrPossibilities.push('error');
         }
 
diff --git a/test/attributes/hx-on.js b/test/attributes/hx-on.js
index 51b4a13b..209366b6 100644
--- a/test/attributes/hx-on.js
+++ b/test/attributes/hx-on.js
@@ -139,7 +139,8 @@ describe("hx-on attribute", function() {
 
     it("can handle event types with dots", function () {
         var btn = make("<button hx-on='my.custom.event: window.foo = true'>Foo</button>");
-        btn.dispatchEvent(new CustomEvent('my.custom.event'));
+        // IE11 doesn't support `new CustomEvent()` so call htmx' internal utility function
+        btn.dispatchEvent(htmx._("makeEvent")('my.custom.event'));
         window.foo.should.equal(true);
         delete window.foo;
     });
diff --git a/test/core/extensions.js b/test/core/extensions.js
index 2bcfe208..f2d3ed2a 100644
--- a/test/core/extensions.js
+++ b/test/core/extensions.js
@@ -29,6 +29,11 @@ describe('Core htmx extension tests', function() {
             onEvent: function(name, evt) {
                 if (name === 'htmx:beforeRequest') {
                     evt.preventDefault();
+                    if (IsIE11()) {
+                        // IE11 doesn't set defaultPrevented to true on custom events it seems, so use a
+                        // return false instead to cancel the event
+                        return false
+                    }
                 }
             }
         });
diff --git a/test/index.html b/test/index.html
index b0841d25..eb6f15b1 100644
--- a/test/index.html
+++ b/test/index.html
@@ -168,7 +168,7 @@
 
 <script class="mocha-exec">
     document.addEventListener("DOMContentLoaded", function () {
-        mocha.setup({globals: ['$0', '$1', '$2', '$3', '$4', 'performance', 'requestAnimationFrame', 'cancelAnimationFrame']}); <!-- IE11 -->
+        mocha.setup({globals: ['$0', '$1', '$2', '$3', '$4', 'performance', 'requestAnimationFrame', 'cancelAnimationFrame', 'confirm']}); <!-- IE11 -->
         mocha.run();
     })
 </script>