diff --git a/src/renderer.js b/src/renderer.js
index 833b8b5..e28dd6d 100644
--- a/src/renderer.js
+++ b/src/renderer.js
@@ -381,7 +381,7 @@ class TabManager {
         try {
             // Check if libraries are available
             if (!marked || !DOMPurify) {
-                preview.innerHTML = '<p style="color: red; padding: 20px;">Error: Required libraries (marked/DOMPurify) not loaded. Check internet connection.</p>';
+                preview.innerHTML = '<div class="preview-error"><div class="preview-error-icon">⚠️</div><div class="preview-error-title">Libraries Not Loaded</div><div class="preview-error-message">Required libraries (marked/DOMPurify) could not be loaded. Please check your installation.</div></div>';
                 return;
             }
             const html = marked.parse(tab.content);
diff --git a/src/styles-modern.css b/src/styles-modern.css
index f38a7ef..5bb51c4 100644
--- a/src/styles-modern.css
+++ b/src/styles-modern.css
@@ -39,13 +39,7 @@
   --transition-ease: cubic-bezier(0.4, 0, 0.2, 1);
 }
 
-/* Reset & Base */
-* {
-  margin: 0;
-  padding: 0;
-  box-sizing: border-box;
-}
-
+/* Base styles - Reset is in styles.css */
 body {
   font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
   overflow: hidden;
diff --git a/src/styles-sidebar.css b/src/styles-sidebar.css
index c32aad6..6b3f36a 100644
--- a/src/styles-sidebar.css
+++ b/src/styles-sidebar.css
@@ -57,6 +57,11 @@
     box-shadow: inset 3px 0 0 var(--primary-dark, #5661b3);
 }
 
+.sidebar-icon:focus-visible {
+    outline: 2px solid var(--primary-dark, #5661b3);
+    outline-offset: 2px;
+}
+
 .sidebar-panel {
     width: 280px;
     background: var(--gray-50, #f9fafb);
diff --git a/src/styles.css b/src/styles.css
index acd1123..9f48055 100644
--- a/src/styles.css
+++ b/src/styles.css
@@ -277,6 +277,165 @@ body {
     line-height: 1.6;
 }
 
+/* ============================================
+   Error & Loading State Components
+   ============================================ */
+
+/* Preview Error State */
+.preview-error {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    padding: 40px 20px;
+    color: #dc3545;
+    text-align: center;
+    min-height: 200px;
+}
+
+.preview-error-icon {
+    font-size: 48px;
+    margin-bottom: 16px;
+    opacity: 0.8;
+}
+
+.preview-error-title {
+    font-size: 18px;
+    font-weight: 600;
+    margin-bottom: 8px;
+}
+
+.preview-error-message {
+    font-size: 14px;
+    color: #666;
+    max-width: 400px;
+    line-height: 1.5;
+}
+
+/* Loading State */
+.loading-spinner {
+    display: inline-block;
+    width: 20px;
+    height: 20px;
+    border: 2px solid var(--gray-200, #e5e7eb);
+    border-top-color: var(--primary-dark, #5661b3);
+    border-radius: 50%;
+    animation: spin 0.8s linear infinite;
+}
+
+@keyframes spin {
+    to { transform: rotate(360deg); }
+}
+
+.loading-overlay {
+    position: absolute;
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    background: rgba(255, 255, 255, 0.8);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    flex-direction: column;
+    gap: 12px;
+    z-index: 100;
+}
+
+.loading-text {
+    font-size: 14px;
+    color: var(--gray-600, #4b5563);
+}
+
+/* Skeleton Loading */
+.skeleton {
+    background: linear-gradient(90deg, var(--gray-100, #f3f4f6) 25%, var(--gray-200, #e5e7eb) 50%, var(--gray-100, #f3f4f6) 75%);
+    background-size: 200% 100%;
+    animation: shimmer 1.5s infinite;
+    border-radius: 4px;
+}
+
+@keyframes shimmer {
+    0% { background-position: 200% 0; }
+    100% { background-position: -200% 0; }
+}
+
+.skeleton-text {
+    height: 14px;
+    margin-bottom: 8px;
+}
+
+.skeleton-text:last-child {
+    width: 60%;
+}
+
+/* Success State */
+.success-message {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    padding: 12px 16px;
+    background: #d1fae5;
+    color: #065f46;
+    border-radius: 6px;
+    font-size: 14px;
+}
+
+.success-message svg {
+    width: 18px;
+    height: 18px;
+    flex-shrink: 0;
+}
+
+/* Warning State */
+.warning-message {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    padding: 12px 16px;
+    background: #fef3c7;
+    color: #92400e;
+    border-radius: 6px;
+    font-size: 14px;
+}
+
+.warning-message svg {
+    width: 18px;
+    height: 18px;
+    flex-shrink: 0;
+}
+
+/* Info State */
+.info-message {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    padding: 12px 16px;
+    background: #dbeafe;
+    color: #1e40af;
+    border-radius: 6px;
+    font-size: 14px;
+}
+
+.info-message svg {
+    width: 18px;
+    height: 18px;
+    flex-shrink: 0;
+}
+
+/* Dark theme adjustments for states */
+body[class*="dark"] .preview-error-message {
+    color: #9ca3af;
+}
+
+body[class*="dark"] .loading-overlay {
+    background: rgba(0, 0, 0, 0.8);
+}
+
+body[class*="dark"] .loading-text {
+    color: #9ca3af;
+}
+
 /* Bottom Panel (REPL Output) */
 .bottom-panel { display: flex; flex-direction: column; border-top: 1px solid var(--gray-200, #e5e7eb); flex-shrink: 0; }
 .bottom-panel.collapsed { height: 0; overflow: hidden; border: none; }