diff --git a/background/style-manager.js b/background/style-manager.js
index 35ac165b..3946cd56 100644
--- a/background/style-manager.js
+++ b/background/style-manager.js
@@ -1,10 +1,16 @@
 /* eslint no-eq-null: 0, eqeqeq: [2, "smart"] */
-/*
-  global createCache db calcStyleDigest normalizeStyleSections db promisify
-  getStyleWithNoCode msg
-*/
+/* global createCache db calcStyleDigest normalizeStyleSections db promisify
+  getStyleWithNoCode msg */
 'use strict';
 
+/*
+This style manager is a layer between content script and the DB. When a style
+is added/updated, it broadcast a message to content script and the content
+script would try to fetch the new code.
+
+The live preview feature relies on `runtime.connect` and `port.onDisconnect`
+to cleanup the temporary code. See /edit/live-preview.js.
+*/
 const styleManager = (() => {
   const preparing = prepare();
   const styles = new Map();
@@ -13,29 +19,7 @@ const styleManager = (() => {
   const compiledExclusion = createCache();
   const BAD_MATCHER = {test: () => false};
 
-  // setup live preview
-  chrome.runtime.onConnect(port => {
-    if (port.name !== 'livePreview') {
-      return;
-    }
-    let id;
-    port.onMessage.addListener(data => {
-      if (!id) {
-        id = data.id;
-      }
-      const style = styles.get(id);
-      style.preview = data;
-      broadcastStyleUpdated(data, 'editPreview');
-    });
-    port.onDisconnect.addListener(() => {
-      port = null;
-      if (id) {
-        const style = styles.get(id);
-        style.preview = null;
-        broadcastStyleUpdated(style.data, 'editPreview');
-      }
-    });
-  });
+  handleLivePreviewConnections();
 
   return ensurePrepared({
     get,
@@ -52,6 +36,31 @@ const styleManager = (() => {
     countStylesByUrl, // used by icon badge
   });
 
+  function handleLivePreviewConnections() {
+    chrome.runtime.onConnect(port => {
+      if (port.name !== 'livePreview') {
+        return;
+      }
+      let id;
+      port.onMessage.addListener(data => {
+        if (!id) {
+          id = data.id;
+        }
+        const style = styles.get(id);
+        style.preview = data;
+        broadcastStyleUpdated(style.preview, 'editPreview');
+      });
+      port.onDisconnect.addListener(() => {
+        port = null;
+        if (id) {
+          const style = styles.get(id);
+          style.preview = null;
+          broadcastStyleUpdated(style.data, 'editPreview');
+        }
+      });
+    });
+  }
+
   function get(id) {
     return styles.get(id).data;
   }
diff --git a/edit/live-preview.js b/edit/live-preview.js
index 7b53b52a..8cb3e7d8 100644
--- a/edit/live-preview.js
+++ b/edit/live-preview.js
@@ -1,16 +1,13 @@
+/* global editors messageBox */
 'use strict';
 
-function createLivePreview() {
+function createLivePreview(preprocess) {
   let data;
   let previewer;
-  let hidden;
-  let node;
-  document.addEventListener('DOMContentLoaded', () => {
-    node = $('#preview-label');
-    if (hidden !== undefined) {
-      node.classList.toggle('hidden', hidden);
-    }
-  }, {once: true});
+  let enabled = prefs.get('editor.livePreview');
+  const label = $('#preview-label');
+  const errorContainer = $('#preview-errors');
+
   prefs.subscribe(['editor.livePreview'], (key, value) => {
     if (value && data && data.id && data.enabled) {
       previewer = createPreviewer;
@@ -20,23 +17,18 @@ function createLivePreview() {
       previewer.disconnect();
       previewer = null;
     }
+    enabled = value;
   });
   return {update, show};
 
   function show(state) {
-    if (hidden === !state) {
-      return;
-    }
-    hidden = !state;
-    if (node) {
-      node.classList.toggle('hidden', hidden);
-    }
+    label.classList.toggle('hidden', !state);
   }
 
   function update(_data) {
     data = _data;
     if (!previewer) {
-      if (!data.id || !data.enabled) {
+      if (!data.id || !data.enabled || !enabled) {
         return;
       }
       previewer = createPreviewer();
@@ -54,7 +46,23 @@ function createLivePreview() {
     return {update, disconnect};
 
     function update(data) {
-      port.postMessage(data);
+      Promise.resolve()
+        .then(() => preprocess ? preprocess(data) : data)
+        .then(data => port.postMessage(data))
+        .then(
+          () => errorContainer.classList.add('hidden'),
+          err => {
+            if (Array.isArray(err)) {
+              err = err.join('\n');
+            } else if (err && err.index !== undefined) {
+              // FIXME: this would fail if editors[0].getValue() !== data.sourceCode
+              const pos = editors[0].posFromIndex(err.index);
+              err.message = `${pos.line}:${pos.ch} ${err.message || String(err)}`;
+            }
+            errorContainer.classList.remove('hidden');
+            errorContainer.onclick = () => messageBox.alert(err.message || String(err), 'pre');
+          }
+        );
     }
 
     function disconnect() {
diff --git a/edit/source-editor.js b/edit/source-editor.js
index 75bca459..d876d5a4 100644
--- a/edit/source-editor.js
+++ b/edit/source-editor.js
@@ -37,7 +37,7 @@ function createSourceEditor(style) {
 
   editors.push(cm);
 
-  const livePreview = createLivePreview();
+  const livePreview = createLivePreview(preprocess);
   livePreview.show(Boolean(style.id));
 
   $('#enabled').onchange = function () {
@@ -92,20 +92,24 @@ function createSourceEditor(style) {
     });
   });
 
-  function updateLivePreview() {
-    if (!style.id) {
-      return;
-    }
-    API.buildUsercss({
-      sourceCode: cm.getValue(),
+  function preprocess(style) {
+    return API.buildUsercss({
+      sourceCode: style.sourceCode,
       vars: style.usercssData.vars
     })
       .then(newStyle => {
         delete newStyle.enabled;
-        livePreview.update(Object.assign({}, style, newStyle));
+        return Object.assign(style, newStyle);
       });
   }
 
+  function updateLivePreview() {
+    if (!style.id) {
+      return;
+    }
+    livePreview.update(Object.assign({}, style, {sourceCode: cm.getValue()}));
+  }
+
   function initAppliesToLineWidget() {
     const PREF_NAME = 'editor.appliesToLineWidget';
     const widget = createAppliesToLineWidget(cm);