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diff --git a/css/04-external-css/index.html b/css/04-external-css/index.html
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+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1.0">
+  <title>Reusable CSS Demo</title>
+  <!-- Linking the separate CSS file -->
+  <link rel="stylesheet" href="styles.css">
+</head>
+<body>
+
+  <h1>Reusable Components Example</h1>
+
+  <!-- Reusing the .card class and different button variations -->
+  <div class="card">
+    <h2>Product Feature A</h2>
+    <p>This layout uses the primary action button style.</p>
+    <button class="btn btn-primary">Accept</button>
+  </div>
+
+  <div class="card">
+    <h2>Product Feature B</h2>
+    <p>This layout reuses the exact same card wrapper but switches to a secondary button.</p>
+    <button class="btn btn-secondary">Learn More</button>
+  </div>
+
+  <div class="card">
+    <h2>The beauty of CSS design</h2>
+    <p>Visit <a href="https://csszengarden.com/" target="_blank">CSS Zen Garden</a> and try changing designs.</p>
+  </div>
+</body>
+</html>
+
diff --git a/css/04-external-css/styles.css b/css/04-external-css/styles.css
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+/* 1. Global design tokens (Variables) */
+:root {
+  --primary-color: #3498db;
+  --secondary-color: #2ecc71;
+  --text-dark: #2c3e50;
+  --font-main: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
+  --border-radius: 8px;
+}
+
+/* 2. Base Element Resets */
+body {
+  font-family: var(--font-main);
+  color: var(--text-dark);
+  line-height: 1.6;
+  padding: 20px;
+}
+
+/* 3. Reusable Component Classes */
+.card {
+  border: 1px solid #e0e0e0;
+  border-radius: var(--border-radius);
+  padding: 20px;
+  margin-bottom: 15px;
+  box-shadow: 3px 3px 10px rgba(0, 0, 0, 0.3);
+}
+
+.btn {
+  display: inline-block;
+  padding: 10px 20px;
+  font-size: 1rem;
+  font-weight: bold;
+  text-align: center;
+  text-decoration: none;
+  border-radius: var(--border-radius);
+  border: none;
+  cursor: pointer;
+  transition: opacity 0.2s ease;
+}
+
+.btn:hover {
+  opacity: 0.9;
+}
+
+/* 4. Reusable Modifier Classes */
+.btn-primary {
+  background-color: var(--primary-color);
+  color: white;
+}
+
+.btn-secondary {
+  background-color: var(--secondary-color);
+  color: white;
+}
+