diff --git a/core/server/apps/default-cards/cards/hr.js b/core/server/apps/default-cards/cards/hr.js
index 3a85026b59..06022a2a19 100644
--- a/core/server/apps/default-cards/cards/hr.js
+++ b/core/server/apps/default-cards/cards/hr.js
@@ -1,5 +1,5 @@
 module.exports = {
-    name: 'hr-card',
+    name: 'card-hr',
     type: 'dom',
     render(opts) {
         return opts.env.dom.createElement('hr');
diff --git a/core/server/apps/default-cards/cards/html.js b/core/server/apps/default-cards/cards/html.js
index b600945dd6..57a2126b93 100644
--- a/core/server/apps/default-cards/cards/html.js
+++ b/core/server/apps/default-cards/cards/html.js
@@ -3,10 +3,10 @@ var SimpleDom   = require('simple-dom'),
     parser;
 
 module.exports = {
-        name: 'html-card',
+        name: 'card-html',
         type: 'dom',
         render(opts) {
             parser = new SimpleDom.HTMLParser(tokenizer, opts.env.dom, SimpleDom.voidMap);
-            return parser.parse('<div>' + opts.payload.html + '</div>');
+            return parser.parse('<div class="kg-card-html">' + opts.payload.html + '</div>');
         }
     };
diff --git a/core/server/apps/default-cards/cards/image.js b/core/server/apps/default-cards/cards/image.js
index 644d580cd5..554f06d5a9 100644
--- a/core/server/apps/default-cards/cards/image.js
+++ b/core/server/apps/default-cards/cards/image.js
@@ -1,8 +1,9 @@
 module.exports = {
-    name: 'image-card',
+    name: 'card-image',
     type: 'dom',
     render(opts) {
         var img = opts.env.dom.createElement('img');
+        img.className = 'kg-card-image';
         img.setAttribute('src', opts.payload.img);
         return img;
     }
diff --git a/core/server/apps/default-cards/cards/markdown.js b/core/server/apps/default-cards/cards/markdown.js
index 7eb5f75c5d..95a30c0be1 100644
--- a/core/server/apps/default-cards/cards/markdown.js
+++ b/core/server/apps/default-cards/cards/markdown.js
@@ -5,10 +5,10 @@ var SimpleDom   = require('simple-dom'),
     parser;
 
 module.exports = {
-        name: 'markdown-card',
+        name: 'card-markdown',
         type: 'dom',
         render(opts) {
             parser = new SimpleDom.HTMLParser(tokenizer, opts.env.dom, SimpleDom.voidMap);
-            return parser.parse('<div>' + converter.makeHtml(opts.payload.markdown || '') + '</div>');
+            return parser.parse('<div class="kg-card-markdown">' + converter.makeHtml(opts.payload.markdown || '') + '</div>');
         }
     };
diff --git a/core/server/apps/default-cards/tests/html_spec.js b/core/server/apps/default-cards/tests/html_spec.js
index 0be91570d0..b2a706a68e 100644
--- a/core/server/apps/default-cards/tests/html_spec.js
+++ b/core/server/apps/default-cards/tests/html_spec.js
@@ -15,7 +15,7 @@ describe('HTML card', function () {
         };
 
         var serializer = new SimpleDom.HTMLSerializer([]);
-        serializer.serialize(card.render(opts)).should.match('<div><h1>HEADING</h1><p>PARAGRAPH</p></div>');
+        serializer.serialize(card.render(opts)).should.match('<div class="kg-card-html"><h1>HEADING</h1><p>PARAGRAPH</p></div>');
     });
     it('Plain content renders', function () {
         opts = {
@@ -28,7 +28,7 @@ describe('HTML card', function () {
         };
 
         var serializer = new SimpleDom.HTMLSerializer([]);
-        serializer.serialize(card.render(opts)).should.match('<div>CONTENT</div>');
+        serializer.serialize(card.render(opts)).should.match('<div class="kg-card-html">CONTENT</div>');
     });
     it.skip('Invalid HTML returns', function () {
         opts = {
@@ -41,6 +41,6 @@ describe('HTML card', function () {
         };
 
         var serializer = new SimpleDom.HTMLSerializer([]);
-        serializer.serialize(card.render(opts)).should.match('<div><h1>HEADING<</div>');
+        serializer.serialize(card.render(opts)).should.match('<div class="kg-card-html"><h1>HEADING<</div>');
     });
 });
diff --git a/core/server/apps/default-cards/tests/markdown_spec.js b/core/server/apps/default-cards/tests/markdown_spec.js
index a952bc4fae..f2f1a29f6e 100644
--- a/core/server/apps/default-cards/tests/markdown_spec.js
+++ b/core/server/apps/default-cards/tests/markdown_spec.js
@@ -15,6 +15,6 @@ describe('Markdown card', function () {
         };
 
         var serializer = new SimpleDom.HTMLSerializer([]);
-        serializer.serialize(card.render(opts)).should.match('<div><h1 id="heading">HEADING</h1>\n\n<ul>\n<li>list</li>\n<li>items</li>\n</ul></div>');
+        serializer.serialize(card.render(opts)).should.match('<div class="kg-card-markdown"><h1 id="heading">HEADING</h1>\n\n<ul>\n<li>list</li>\n<li>items</li>\n</ul></div>');
     });
 });
diff --git a/core/server/utils/mobiledoc-converter.js b/core/server/utils/mobiledoc-converter.js
index 248c6e447c..8940b2784b 100644
--- a/core/server/utils/mobiledoc-converter.js
+++ b/core/server/utils/mobiledoc-converter.js
@@ -1,11 +1,17 @@
 var SimpleDom   = require('simple-dom'),
     Renderer    = require('mobiledoc-dom-renderer').default,
     config      = require('../config'),
+    logging     = require('../logging'),
+    errors      = require('../errors'),
     defaults    = require(config.get('paths').internalAppPath + 'default-cards'),
     options = {
         dom: new SimpleDom.Document(),
         cards: defaults.cards,
-        atoms: defaults.atoms
+        atoms: defaults.atoms,
+        unknownCardHandler: function (args) {
+            var error = new errors.InternalServerError({message: 'Mobiledoc card \'' + args.env.name + '\' not found.'});
+            logging.error(error);
+        }
     };
 
 // function getCards() {
diff --git a/core/test/unit/utils/mobiledoc-converter_spec.js b/core/test/unit/utils/mobiledoc-converter_spec.js
index f6f000eb0e..ce2712dc1f 100644
--- a/core/test/unit/utils/mobiledoc-converter_spec.js
+++ b/core/test/unit/utils/mobiledoc-converter_spec.js
@@ -6,15 +6,17 @@ describe('Convert mobiledoc to HTML ', function () {
         version: '0.3.1',
         atoms: [],
         cards: [
-            ['markdown-card',
+            ['card-markdown',
                 {
                     pos: 'top',
-                    card_name: 'markdown-card',
+                    card_name: 'card-markdown',
                     markdown: '#heading\n\n- list one\n- list two\n- list three'
                 }
             ],
-            ['markdown-card', {
-                pos: 'top'
+            ['card-html', {
+                pos: 'top',
+                card_name: 'card-html',
+                html: '<p>HTML CARD</p>'
             }]
         ],
         markups: [],
@@ -27,6 +29,6 @@ describe('Convert mobiledoc to HTML ', function () {
         ]
     };
     it('Converts a mobiledoc to HTML', function () {
-        converter.render(mobiledoc).should.match('<p>test</p><div><h1 id="heading">heading</h1>\n\n<ul>\n<li>list one</li>\n<li>list two</li>\n<li>list three</li>\n</ul></div><div></div>');
+        converter.render(mobiledoc).should.match('<p>test</p><div class="kg-card-markdown"><h1 id="heading">heading</h1>\n\n<ul>\n<li>list one</li>\n<li>list two</li>\n<li>list three</li>\n</ul></div><div class="kg-card-html"><p>HTML CARD</p></div>');
     });
 });