:root {
            --primary-blue: #3b82f6;
            --primary-dark: #1e40af;
            --secondary-blue: #60a5fa;
            --accent-blue: #93c5fd;
            --light-blue: #eff6ff;
            --gradient-start: #667eea;
            --gradient-end: #764ba2;
            --text-dark: #1f2937;
            --text-medium: #4b5563;
            --text-light: #6b7280;
            --surface: #ffffff;
            --surface-hover: #f8fafc;
            --border: #e5e7eb;
            --success: #10b981;
            --warning: #f59e0b;
            --error: #ef4444;
            --shadow-sm: 0 1px 2px 0 rgb(0 0 0 / 0.05);
            --shadow-md: 0 4px 6px -1px rgb(0 0 0 / 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1);
            --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.1);
            --shadow-xl: 0 20px 25px -5px rgb(0 0 0 / 0.1), 0 8px 10px -6px rgb(0 0 0 / 0.1);
        }

        * { box-sizing: border-box; margin: 0; padding: 0; }

        body {
            font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
            line-height: 1.6;
            color: var(--text-dark);
            background: linear-gradient(135deg, #f0f9ff 0%, #e0e7ff 100%);
            min-height: 100vh;
            overflow-x: hidden;
        }

        .container { max-width: 1000px; margin: 0 auto; padding: 2rem; position: relative; }

        .bg-decoration { position: fixed; pointer-events: none; z-index: -1; opacity: 0.1; }
        .bg-wave-1 { top: 10%; right: -10%; width: 300px; height: 300px; background: radial-gradient(circle, var(--primary-blue) 0%, transparent 70%); border-radius: 50%; animation: float 20s ease-in-out infinite; }
        .bg-wave-2 { bottom: 10%; left: -15%; width: 400px; height: 400px; background: radial-gradient(circle, var(--secondary-blue) 0%, transparent 70%); border-radius: 50%; animation: float 25s ease-in-out infinite reverse; }
        @keyframes float { 0%, 100% { transform: translateY(0px) rotate(0deg); } 50% { transform: translateY(-20px) rotate(5deg); } }

        h1 {
            font-size: clamp(2.5rem, 5vw, 4rem); font-weight: 700;
            background: linear-gradient(135deg, var(--gradient-start), var(--gradient-end));
            -webkit-background-clip: text; -webkit-text-fill-color: transparent; background-clip: text;
            text-align: center; margin-bottom: 3rem; position: relative; animation: fadeInUp 1s ease-out;
        }
        h1::after { content: ''; position: absolute; bottom: -10px; left: 50%; transform: translateX(-50%); width: 100px; height: 4px; background: linear-gradient(90deg, var(--gradient-start), var(--gradient-end)); border-radius: 2px; animation: scaleXAnim 1s ease-out 0.5s both; }
        @keyframes fadeInUp { from { opacity: 0; transform: translateY(30px); } to { opacity: 1; transform: translateY(0); } }
        @keyframes scaleXAnim { from { transform: translateX(-50%) scaleX(0); } to { transform: translateX(-50%) scaleX(1); } }

        h2 { color: var(--primary-dark); font-size: 1.875rem; font-weight: 600; margin-bottom: 1.5rem; position: relative; padding-left: 1rem; }
        h2::before { content: ''; position: absolute; left: 0; top: 50%; transform: translateY(-50%); width: 4px; height: 60%; background: linear-gradient(180deg, var(--primary-blue), var(--secondary-blue)); border-radius: 2px; }
        h3 { color: var(--primary-blue); font-size: 1.5rem; font-weight: 600; margin: 2rem 0 1rem 0; }
        h4 { color: var(--primary-dark); font-size: 1.125rem; font-weight: 500; margin-bottom: 0.75rem; }

        p { margin-bottom: 0.75rem; }
        ul { margin: 0.5rem 0 1rem 1.5rem; }
        li { margin-bottom: 0.25rem; }

        .card {
            background: var(--surface); border-radius: 16px; padding: 2rem; margin: 2rem 0;
            box-shadow: var(--shadow-lg); border: 1px solid var(--border); position: relative;
            overflow: hidden; transition: all 0.3s ease; animation: slideInUp 0.6s ease-out;
        }
        .card::before { content: ''; position: absolute; top: 0; left: 0; right: 0; height: 4px; background: linear-gradient(90deg, var(--gradient-start), var(--gradient-end)); }
        .card:hover { transform: translateY(-3px); box-shadow: var(--shadow-xl); }
        @keyframes slideInUp { from { opacity: 0; transform: translateY(50px); } to { opacity: 1; transform: translateY(0); } }

        .formula {
            background: linear-gradient(135deg, var(--light-blue), #f0f9ff); border: 2px solid var(--accent-blue);
            border-radius: 12px; padding: 1.5rem; margin: 1.5rem 0; text-align: center;
            font-size: 1.125rem; font-weight: 500; position: relative; transition: all 0.3s ease;
        }
        .formula:hover { transform: scale(1.02); box-shadow: var(--shadow-md); }
        .formula::before { content: '\1F4D0'; position: absolute; top: -10px; left: 20px; background: var(--surface); padding: 0 8px; border-radius: 50%; font-size: 1.5rem; }

        .interactive {
            background: linear-gradient(135deg, #fefce8, #fef3c7); border: 2px solid #fbbf24;
            border-radius: 12px; padding: 1.5rem; margin: 2rem 0; position: relative;
        }
        .interactive::before { content: '\26A1'; position: absolute; top: -10px; right: 20px; background: var(--surface); padding: 0 8px; border-radius: 50%; font-size: 1.5rem; }

        .responsive-flex { display: grid; grid-template-columns: repeat(auto-fit, minmax(280px, 1fr)); gap: 1.5rem; margin: 1rem 0; }
        .flex-item { display: flex; flex-direction: column; }

        label { font-weight: 500; color: var(--text-medium); margin-bottom: 0.5rem; font-size: 0.875rem; text-transform: uppercase; letter-spacing: 0.05em; }
        input[type="number"], select { padding: 0.75rem 1rem; border: 2px solid var(--border); border-radius: 8px; font-size: 1rem; transition: all 0.3s ease; background: var(--surface); margin-bottom: 1rem; }
        input[type="number"]:focus, select:focus { outline: none; border-color: var(--primary-blue); box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.1); }

        button {
            background: linear-gradient(135deg, var(--primary-blue), var(--primary-dark)); color: white;
            border: none; padding: 0.75rem 1.5rem; border-radius: 8px; font-size: 1rem;
            font-weight: 500; cursor: pointer; transition: all 0.3s ease; position: relative; overflow: hidden;
        }
        button::before { content: ''; position: absolute; top: 0; left: -100%; width: 100%; height: 100%; background: linear-gradient(90deg, transparent, rgba(255,255,255,0.2), transparent); transition: left 0.5s; }
        button:hover::before { left: 100%; }
        button:hover { transform: translateY(-2px); box-shadow: var(--shadow-lg); }
        button:active { transform: translateY(0); }

        .result { background: linear-gradient(135deg, #f0fdf4, #dcfce7); border: 2px solid var(--success); border-radius: 12px; padding: 1.5rem; margin-top: 1rem; font-weight: 500; line-height: 1.6; }

        .learning-outcomes { background: linear-gradient(135deg, var(--light-blue), #dbeafe); border: 2px solid var(--accent-blue); border-radius: 12px; padding: 1.5rem; margin: 1.5rem 0; position: relative; }
        .learning-outcomes::before { content: '\1F3AF'; position: absolute; top: -10px; left: 20px; background: var(--surface); padding: 0 8px; border-radius: 50%; font-size: 1.5rem; }

        .definition-term { font-weight: 600; color: var(--primary-blue); text-decoration: underline; text-decoration-color: var(--accent-blue); text-decoration-thickness: 2px; text-underline-offset: 2px; }

        .example { background: linear-gradient(135deg, #fffbeb, #fef3c7); border: 2px solid #f59e0b; border-radius: 12px; padding: 1.5rem; margin: 1.5rem 0; border-left: 6px solid var(--warning); position: relative; }
        .example::before { content: '\1F4A1'; position: absolute; top: -10px; right: 20px; background: var(--surface); padding: 0 8px; border-radius: 50%; font-size: 1.5rem; }

        .explanation { font-style: italic; color: var(--text-light); padding: 1rem; background: linear-gradient(135deg, #f9fafb, #f3f4f6); border-radius: 8px; border-left: 4px solid var(--accent-blue); margin: 1rem 0; }

        /* ===== Simulation Styles ===== */
        .sim-container {
            background: var(--surface); border: 2px solid var(--border); border-radius: 16px;
            padding: 1.5rem; margin: 2rem 0; text-align: center; box-shadow: var(--shadow-md);
            position: relative; overflow: hidden;
        }
        .sim-container::before { content: ''; position: absolute; top: 0; left: 0; right: 0; height: 4px; background: linear-gradient(90deg, #667eea, #764ba2); }
        .sim-container h4 { margin-bottom: 0.5rem; }

        canvas {
            display: block; margin: 0.75rem auto; border-radius: 10px;
            background: linear-gradient(180deg, #f8fafc, #f1f5f9);
            border: 1px solid #e2e8f0; max-width: 100%;
        }

        .sim-controls {
            display: flex; flex-wrap: wrap; gap: 1rem; margin-top: 1rem;
            padding: 1rem; background: linear-gradient(135deg, #f8fafc, #eef2ff);
            border-radius: 10px; justify-content: center; align-items: flex-start;
        }
        .sim-control { flex: 1; min-width: 160px; max-width: 250px; text-align: left; }
        .sim-control label { display: block; font-size: 0.8rem; margin-bottom: 0.3rem; text-transform: none; letter-spacing: normal; font-weight: 600; color: var(--text-medium); }
        .sim-control .val { color: var(--primary-blue); font-weight: 700; }

        input[type="range"] {
            -webkit-appearance: none; appearance: none; width: 100%; height: 6px;
            border-radius: 3px; background: #ddd; outline: none; margin: 0.4rem 0;
            border: none; padding: 0;
        }
        input[type="range"]::-webkit-slider-thumb {
            -webkit-appearance: none; appearance: none; width: 18px; height: 18px;
            border-radius: 50%; background: linear-gradient(135deg, var(--primary-blue), var(--primary-dark));
            cursor: pointer; box-shadow: 0 2px 6px rgba(59, 130, 246, 0.4);
        }
        input[type="range"]::-moz-range-thumb {
            width: 18px; height: 18px; border-radius: 50%;
            background: linear-gradient(135deg, var(--primary-blue), var(--primary-dark));
            cursor: pointer; border: none; box-shadow: 0 2px 6px rgba(59, 130, 246, 0.4);
        }

        .btn-row { display: flex; gap: 0.75rem; justify-content: center; margin-top: 1rem; flex-wrap: wrap; }
        .btn-secondary { background: linear-gradient(135deg, #6b7280, #4b5563); }

        /* Quiz Styles */
        .quiz-container { background: linear-gradient(135deg, #f8fafc, #e2e8f0); border: 2px solid #94a3b8; border-radius: 16px; padding: 2rem; position: relative; }
        .quiz-container::before { content: '\1F4DD'; position: absolute; top: -10px; right: 20px; background: var(--surface); padding: 0 8px; border-radius: 50%; font-size: 1.5rem; }
        .quiz-question { background: var(--surface); border-radius: 12px; padding: 1.5rem; margin-bottom: 1.5rem; box-shadow: var(--shadow-sm); transition: all 0.3s ease; }
        .quiz-question:hover { box-shadow: var(--shadow-md); }
        .quiz-question input[type="radio"] { margin-right: 0.75rem; transform: scale(1.2); accent-color: var(--primary-blue); }
        .quiz-question label { font-size: 1rem; font-weight: 400; text-transform: none; letter-spacing: normal; cursor: pointer; padding: 0.5rem; border-radius: 6px; transition: background-color 0.2s ease; }
        .quiz-question label:hover { background-color: var(--surface-hover); }
        .quiz-feedback { margin-top: 0.5rem; padding: 0.5rem 0.75rem; border-radius: 6px; font-size: 0.9rem; display: none; }
        .quiz-feedback.correct { display: block; background: #dcfce7; color: #166534; border: 1px solid #86efac; }
        .quiz-feedback.incorrect { display: block; background: #fef2f2; color: #991b1b; border: 1px solid #fca5a5; }

        /* Responsive */
        @media (max-width: 768px) {
            .container { padding: 1rem; }
            .card { padding: 1.5rem; margin: 1.5rem 0; }
            .responsive-flex { grid-template-columns: 1fr; }
            input[type="number"], select, button { width: 100%; }
            h1 { font-size: 2.25rem; }
            h2 { font-size: 1.5rem; }
            .sim-controls { flex-direction: column; align-items: stretch; }
            .sim-control { max-width: 100%; }
        }

        .reveal { opacity: 0; transform: translateY(50px); transition: all 0.6s ease; }
        .reveal.active { opacity: 1; transform: translateY(0); }