/* NATO Phonetic Design System */
        :root {
            /* Military Color Palette */
            --nato-primary: #1e3a8a;
            --nato-secondary: #1e40af;
            --nato-accent: #dc2626;
            --nato-gold: #f59e0b;
            --nato-green: #059669;
            --nato-dark: #1e293b;
            --nato-light: #f8fafc;
            --nato-blue: #0369a1;
            
            /* Aviation Colors */
            --aviation-blue: #0284c7;
            --aviation-orange: #ea580c;
            --aviation-silver: #e2e8f0;
            --military-green: #166534;
            --signal-red: #dc2626;
            
            /* Surface Colors */
            --bg-primary: #ffffff;
            --bg-secondary: #f8fafc;
            --bg-tertiary: #f1f5f9;
            --surface-elevated: #ffffff;
            --text-primary: #1f2937;
            --text-secondary: #6b7280;
            --border-color: rgba(30, 58, 138, 0.2);
            
            /* Spacing Scale */
            --space-xs: 0.25rem;
            --space-sm: 0.5rem;
            --space-md: 1rem;
            --space-lg: 1.5rem;
            --space-xl: 2rem;
            --space-2xl: 3rem;
            --space-3xl: 4rem;
            
            /* Shadows */
            --shadow-sm: 0 1px 2px 0 rgba(30, 58, 138, 0.05);
            --shadow-md: 0 4px 6px -1px rgba(30, 58, 138, 0.1);
            --shadow-lg: 0 10px 15px -3px rgba(30, 58, 138, 0.1);
            --shadow-xl: 0 20px 25px -5px rgba(30, 58, 138, 0.1);
            
            /* Border Radius */
            --radius: 0.75rem;
            --radius-sm: 0.5rem;
            --radius-lg: 1rem;
            --radius-xl: 1.5rem;
            
            /* Transitions */
            --transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
        }

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

        body {
            font-family: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
            background: linear-gradient(135deg, var(--bg-secondary) 0%, var(--aviation-silver) 50%, var(--nato-light) 100%);
            color: var(--text-primary);
            line-height: 1.6;
            min-height: 100vh;
        }

        /* Hero Section */
        .hero {
            background: linear-gradient(135deg, var(--nato-dark) 0%, var(--nato-primary) 30%, var(--aviation-blue) 70%, var(--nato-secondary) 100%);
            color: white;
            padding: var(--space-3xl) var(--space-lg);
            text-align: center;
            position: relative;
            overflow: hidden;
        }

        .hero::before {
            content: '';
            position: absolute;
            top: 0;
            left: 0;
            right: 0;
            bottom: 0;
            background: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100"><g fill="%23ffffff" opacity="0.08"><path d="M20,20 L30,10 L40,20 L50,10 L60,20 L50,30 L40,20 L30,30 Z"/><circle cx="70" cy="25" r="8" fill="none" stroke="%23ffffff" stroke-width="2"/><rect x="15" y="60" width="20" height="15" fill="none" stroke="%23ffffff" stroke-width="2"/><polygon points="70,70 80,60 90,70 85,80" fill="none" stroke="%23ffffff" stroke-width="2"/></g></svg>') repeat;
            animation: military-drift 30s linear infinite;
            pointer-events: none;
        }

        @keyframes military-drift {
            0% { transform: translateX(0) translateY(0) rotate(0deg); }
            100% { transform: translateX(-100px) translateY(-100px) rotate(360deg); }
        }

        .hero-badge {
            display: inline-flex;
            align-items: center;
            gap: var(--space-sm);
            background: rgba(255, 255, 255, 0.15);
            backdrop-filter: blur(15px);
            color: white;
            padding: var(--space-sm) var(--space-lg);
            border-radius: 2rem;
            font-size: 0.875rem;
            font-weight: 600;
            margin-bottom: var(--space-lg);
            border: 1px solid rgba(255, 255, 255, 0.2);
            box-shadow: 0 8px 32px rgba(0, 0, 0, 0.1);
        }

        .hero-title {
            font-size: clamp(2.5rem, 6vw, 4rem);
            font-weight: 800;
            margin-bottom: var(--space-lg);
            line-height: 1.1;
            text-shadow: 2px 2px 8px rgba(0, 0, 0, 0.3);
            background: linear-gradient(135deg, #ffffff, var(--nato-gold));
            background-clip: text;
            -webkit-background-clip: text;
            -webkit-text-fill-color: transparent;
        }

        .hero-subtitle {
            font-size: 1.25rem;
            opacity: 0.95;
            max-width: 700px;
            margin: 0 auto var(--space-xl);
            line-height: 1.6;
        }

        .difficulty-badge {
            display: inline-flex;
            align-items: center;
            gap: var(--space-xs);
            background: var(--aviation-orange);
            color: white;
            padding: var(--space-sm) var(--space-lg);
            border-radius: var(--radius);
            font-weight: 600;
            font-size: 0.875rem;
            box-shadow: 0 4px 12px rgba(234, 88, 12, 0.3);
        }

        /* Main Container */
        .container {
            max-width: 1400px;
            margin: 0 auto;
            padding: 0 var(--space-lg);
        }

        /* Educational Section */
        .educational-section {
            background: var(--surface-elevated);
            margin: calc(-1 * var(--space-xl)) var(--space-lg) var(--space-2xl);
            border-radius: var(--radius-xl);
            padding: var(--space-2xl);
            box-shadow: var(--shadow-xl);
            border: 1px solid var(--border-color);
            position: relative;
            z-index: 10;
        }

        .section-title {
            font-size: 1.5rem;
            font-weight: 700;
            color: var(--nato-primary);
            margin-bottom: var(--space-lg);
            display: flex;
            align-items: center;
            gap: var(--space-sm);
        }

        .info-grid {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
            gap: var(--space-lg);
            margin-bottom: var(--space-xl);
        }

        .info-card {
            background: linear-gradient(135deg, var(--nato-primary) 0%, var(--aviation-blue) 100%);
            color: white;
            padding: var(--space-xl);
            border-radius: var(--radius-lg);
            text-align: center;
            transform: translateY(0);
            transition: var(--transition);
            position: relative;
            overflow: hidden;
        }

        .info-card::before {
            content: '';
            position: absolute;
            top: -50%;
            left: -50%;
            width: 200%;
            height: 200%;
            background: linear-gradient(45deg, transparent, rgba(255, 255, 255, 0.1), transparent);
            transform: rotate(45deg);
            transition: var(--transition);
            opacity: 0;
        }

        .info-card:hover::before {
            opacity: 1;
            animation: radar-sweep 1.5s ease-in-out;
        }

        @keyframes radar-sweep {
            0% { transform: translateX(-100%) translateY(-100%) rotate(45deg); }
            100% { transform: translateX(100%) translateY(100%) rotate(45deg); }
        }

        .info-card:hover {
            transform: translateY(-6px);
            box-shadow: var(--shadow-xl);
        }

        .info-card-icon {
            font-size: 2.5rem;
            margin-bottom: var(--space-md);
            filter: drop-shadow(0 4px 8px rgba(0, 0, 0, 0.2));
        }

        .info-card-title {
            font-size: 1.25rem;
            font-weight: 700;
            margin-bottom: var(--space-sm);
        }

        .info-card-text {
            font-size: 0.9rem;
            opacity: 0.9;
            line-height: 1.6;
        }

        /* Calculator Layout */
        .calculator-layout {
            display: grid;
            grid-template-columns: 1fr 1fr;
            gap: var(--space-2xl);
            margin-bottom: var(--space-3xl);
        }

        .calculator-panel {
            background: var(--surface-elevated);
            border-radius: var(--radius-xl);
            padding: var(--space-2xl);
            box-shadow: var(--shadow-lg);
            border: 1px solid var(--border-color);
            backdrop-filter: blur(10px);
        }

        .panel-title {
            font-size: 1.5rem;
            font-weight: 700;
            color: var(--nato-primary);
            margin-bottom: var(--space-xl);
            display: flex;
            align-items: center;
            gap: var(--space-sm);
        }

        /* NATO Phonetic Display */
        .nato-display {
            background: linear-gradient(145deg, #f8fafc, #e2e8f0);
            border: 3px solid var(--nato-primary);
            border-radius: var(--radius-lg);
            padding: var(--space-2xl);
            margin: var(--space-lg) 0;
            min-height: 200px;
            position: relative;
            overflow: hidden;
        }

        .nato-display::before {
            content: '';
            position: absolute;
            top: 0;
            left: 0;
            right: 0;
            bottom: 0;
            background: radial-gradient(circle at center, rgba(30, 58, 138, 0.05) 0%, transparent 70%);
            pointer-events: none;
        }

        .phonetic-word {
            display: inline-block;
            background: linear-gradient(135deg, var(--nato-primary), var(--aviation-blue));
            color: white;
            padding: var(--space-sm) var(--space-md);
            margin: var(--space-xs);
            border-radius: var(--radius);
            font-weight: 600;
            font-size: 1rem;
            box-shadow: var(--shadow-md);
            animation: word-appear 0.6s ease-out;
            position: relative;
            z-index: 1;
            cursor: pointer;
            transition: var(--transition);
        }

        .phonetic-word:hover {
            transform: translateY(-2px) scale(1.05);
            box-shadow: var(--shadow-lg);
        }

        .phonetic-word.playing {
            animation: pulse-glow 1s ease-in-out;
            background: linear-gradient(135deg, var(--signal-red), var(--aviation-orange));
        }

        @keyframes word-appear {
            from { 
                opacity: 0; 
                transform: scale(0.8) translateY(20px); 
            }
            to { 
                opacity: 1; 
                transform: scale(1) translateY(0); 
            }
        }

        @keyframes pulse-glow {
            0%, 100% { 
                box-shadow: var(--shadow-md);
                transform: scale(1);
            }
            50% { 
                box-shadow: 0 0 20px rgba(220, 38, 38, 0.6);
                transform: scale(1.1);
            }
        }

        /* Input Controls */
        .input-group {
            margin-bottom: var(--space-xl);
        }

        .input-label {
            display: block;
            font-size: 1rem;
            font-weight: 600;
            color: var(--text-primary);
            margin-bottom: var(--space-sm);
        }

        .input-field {
            width: 100%;
            padding: var(--space-lg);
            border: 2px solid var(--border-color);
            border-radius: var(--radius);
            font-size: 1rem;
            min-height: 120px;
            resize: vertical;
            transition: var(--transition);
            font-family: inherit;
            background: var(--bg-primary);
        }

        .input-field:focus {
            outline: none;
            border-color: var(--nato-primary);
            box-shadow: 0 0 0 3px rgba(30, 58, 138, 0.1);
            background: var(--bg-secondary);
        }

        .output-field {
            background: var(--bg-tertiary);
            border: 2px solid var(--nato-secondary);
            font-size: 1.125rem;
            font-weight: 600;
            color: var(--nato-primary);
            min-height: 120px;
        }

        .output-field:focus {
            border-color: var(--nato-secondary);
        }

        /* Mode Selector */
        .mode-selector {
            display: flex;
            background: var(--bg-secondary);
            border-radius: var(--radius);
            padding: 4px;
            margin-bottom: var(--space-xl);
            border: 1px solid var(--border-color);
            box-shadow: var(--shadow-sm);
        }

        .mode-button {
            flex: 1;
            padding: var(--space-md);
            border: none;
            border-radius: calc(var(--radius) - 4px);
            background: transparent;
            cursor: pointer;
            transition: var(--transition);
            font-weight: 600;
            color: var(--text-secondary);
            position: relative;
            overflow: hidden;
        }

        .mode-button::before {
            content: '';
            position: absolute;
            top: 0;
            left: -100%;
            width: 100%;
            height: 100%;
            background: linear-gradient(90deg, transparent, rgba(255, 255, 255, 0.4), transparent);
            transition: var(--transition);
        }

        .mode-button:hover::before {
            left: 100%;
        }

        .mode-button.active {
            background: linear-gradient(135deg, var(--nato-primary), var(--nato-secondary));
            color: white;
            box-shadow: var(--shadow-md);
            transform: translateY(-1px);
        }

        .mode-button:hover:not(.active) {
            background: rgba(30, 58, 138, 0.1);
            color: var(--nato-primary);
        }

        /* Action Buttons */
        .action-buttons {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(160px, 1fr));
            gap: var(--space-md);
            margin-top: var(--space-lg);
        }

        .btn {
            padding: var(--space-md) var(--space-lg);
            border: none;
            border-radius: var(--radius);
            font-weight: 600;
            cursor: pointer;
            transition: var(--transition);
            display: flex;
            align-items: center;
            justify-content: center;
            gap: var(--space-sm);
            font-size: 0.875rem;
            position: relative;
            overflow: hidden;
        }

        .btn::before {
            content: '';
            position: absolute;
            top: 50%;
            left: 50%;
            width: 0;
            height: 0;
            background: rgba(255, 255, 255, 0.3);
            border-radius: 50%;
            transition: all 0.5s ease;
            transform: translate(-50%, -50%);
        }

        .btn:hover::before {
            width: 300px;
            height: 300px;
        }

        .btn-primary {
            background: linear-gradient(135deg, var(--nato-primary), var(--nato-secondary));
            color: white;
            box-shadow: var(--shadow-md);
        }

        .btn-primary:hover {
            transform: translateY(-2px);
            box-shadow: var(--shadow-lg);
        }

        .btn-secondary {
            background: var(--bg-secondary);
            color: var(--text-primary);
            border: 1px solid var(--border-color);
        }

        .btn-secondary:hover {
            background: rgba(30, 58, 138, 0.1);
            border-color: var(--nato-primary);
        }

        .btn-audio {
            background: linear-gradient(135deg, var(--military-green), var(--nato-green));
            color: white;
            box-shadow: var(--shadow-md);
        }

        .btn-audio:hover {
            transform: translateY(-2px);
            box-shadow: var(--shadow-lg);
        }

        .btn-audio:disabled {
            opacity: 0.6;
            cursor: not-allowed;
            transform: none;
        }

        /* Audio Controls */
        .audio-controls {
            background: linear-gradient(135deg, var(--nato-dark), var(--military-green));
            border-radius: var(--radius-lg);
            padding: var(--space-lg);
            margin-top: var(--space-lg);
            color: white;
            display: flex;
            align-items: center;
            justify-content: space-between;
            box-shadow: var(--shadow-md);
        }

        .audio-status {
            display: flex;
            align-items: center;
            gap: var(--space-sm);
            font-weight: 600;
        }

        .audio-indicator {
            width: 12px;
            height: 12px;
            border-radius: 50%;
            background: var(--signal-red);
            animation: blink 1s infinite;
        }

        .audio-indicator.playing {
            background: var(--nato-green);
            animation: pulse 1s infinite;
        }

        @keyframes blink {
            0%, 50% { opacity: 1; }
            51%, 100% { opacity: 0.3; }
        }

        @keyframes pulse {
            0%, 100% { transform: scale(1); opacity: 1; }
            50% { transform: scale(1.2); opacity: 0.8; }
        }

        .speed-control {
            display: flex;
            align-items: center;
            gap: var(--space-sm);
        }

        .speed-slider {
            width: 100px;
            height: 4px;
            border-radius: 2px;
            background: rgba(255, 255, 255, 0.3);
            outline: none;
            cursor: pointer;
        }

        .speed-slider::-webkit-slider-thumb {
            appearance: none;
            width: 16px;
            height: 16px;
            border-radius: 50%;
            background: white;
            cursor: pointer;
            box-shadow: 0 2px 4px rgba(0, 0, 0, 0.2);
        }

        .speed-slider::-moz-range-thumb {
            width: 16px;
            height: 16px;
            border-radius: 50%;
            background: white;
            cursor: pointer;
            border: none;
            box-shadow: 0 2px 4px rgba(0, 0, 0, 0.2);
        }

        .challenge-option {
            background: rgba(255, 255, 255, 0.2);
            border: 2px solid rgba(255, 255, 255, 0.3);
            border-radius: var(--radius);
            padding: var(--space-md);
            color: white;
            font-weight: 600;
            cursor: pointer;
            transition: var(--transition);
            text-align: center;
        }

        .challenge-option:hover {
            background: rgba(255, 255, 255, 0.3);
            border-color: rgba(255, 255, 255, 0.5);
            transform: translateY(-2px);
        }

        .challenge-option.correct {
            background: rgba(5, 150, 105, 0.8);
            border-color: rgba(5, 150, 105, 1);
            animation: correct-flash 0.6s ease-out;
        }

        .challenge-option.incorrect {
            background: rgba(220, 38, 38, 0.8);
            border-color: rgba(220, 38, 38, 1);
            animation: shake 0.5s ease-out;
        }

        @keyframes correct-flash {
            0% { transform: scale(1); }
            50% { transform: scale(1.1); box-shadow: 0 0 20px rgba(5, 150, 105, 0.6); }
            100% { transform: scale(1); }
        }

        @keyframes shake {
            0%, 100% { transform: translateX(0); }
            25% { transform: translateX(-5px); }
            75% { transform: translateX(5px); }
        }

        /* Responsive Design */
        @media (max-width: 768px) {
            .calculator-layout {
                grid-template-columns: 1fr;
                gap: var(--space-xl);
            }

            .info-grid {
                grid-template-columns: 1fr;
            }

            .examples-grid {
                grid-template-columns: 1fr;
            }

            .alphabet-grid {
                grid-template-columns: repeat(auto-fit, minmax(80px, 1fr));
            }

            .challenge-options {
                grid-template-columns: 1fr;
            }

            .hero {
                padding: var(--space-2xl) var(--space-lg);
            }

            .educational-section {
                margin-left: 0;
                margin-right: 0;
                border-radius: 0;
            }

            .action-buttons {
                grid-template-columns: 1fr;
            }

            .nato-display {
                padding: var(--space-lg);
                min-height: 150px;
            }

            .phonetic-word {
                font-size: 0.875rem;
            }

            .audio-controls {
                flex-direction: column;
                gap: var(--space-md);
                text-align: center;
            }
        }

        /* Animation Classes */
        .fade-in {
            animation: fadeIn 0.6s ease-out;
        }

        @keyframes fadeIn {
            from { opacity: 0; transform: translateY(30px); }
            to { opacity: 1; transform: translateY(0); }
        }

        .bounce {
            animation: bounce 0.6s ease-out;
        }

        @keyframes bounce {
            0%, 100% { transform: translateY(0); }
            50% { transform: translateY(-10px); }
        }

        /* Military Theme Enhancements */
        .signal-strength {
            display: flex;
            align-items: center;
            gap: 4px;
            margin-left: var(--space-sm);
        }

        .signal-bar {
            width: 4px;
            background: var(--nato-green);
            border-radius: 2px;
            transition: var(--transition);
        }

        .signal-bar:nth-child(1) { height: 8px; }
        .signal-bar:nth-child(2) { height: 12px; }
        .signal-bar:nth-child(3) { height: 16px; }
        .signal-bar:nth-child(4) { height: 20px; }

        .transmission-complete {
            background: linear-gradient(135deg, var(--nato-green), var(--military-green));
            border-radius: var(--radius-lg);
            padding: var(--space-lg);
            margin-top: var(--space-lg);
            color: white;
            text-align: center;
            font-weight: 600;
            animation: transmission-pulse 2s ease-in-out;
        }

        @keyframes transmission-pulse {
            0%, 100% { 
                box-shadow: 0 0 0 0 rgba(5, 150, 105, 0.7);
                transform: scale(1);
            }
            50% { 
                box-shadow: 0 0 20px 10px rgba(5, 150, 105, 0);
                transform: scale(1.02);
            }
        }

        /* Alphabet Reference Section */
        .alphabet-section {
            background: var(--surface-elevated);
            border-radius: var(--radius-xl);
            padding: var(--space-2xl);
            margin-bottom: var(--space-2xl);
            box-shadow: var(--shadow-lg);
            border: 1px solid var(--border-color);
        }

        .alphabet-grid {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(120px, 1fr));
            gap: var(--space-md);
            margin-top: var(--space-lg);
        }

        .alphabet-card {
            background: linear-gradient(145deg, var(--bg-secondary), var(--aviation-silver));
            border: 2px solid var(--nato-gold);
            border-radius: var(--radius);
            padding: var(--space-lg);
            text-align: center;
            transition: var(--transition);
            cursor: pointer;
            position: relative;
            overflow: hidden;
        }

        .alphabet-card::before {
            content: '';
            position: absolute;
            top: 0;
            left: -100%;
            width: 100%;
            height: 100%;
            background: linear-gradient(90deg, transparent, rgba(245, 158, 11, 0.3), transparent);
            transition: var(--transition);
        }

        .alphabet-card:hover::before {
            left: 100%;
        }

        .alphabet-card:hover {
            transform: translateY(-3px) scale(1.05);
            box-shadow: var(--shadow-md);
            border-color: var(--nato-primary);
        }

        .alphabet-letter {
            font-size: 1.5rem;
            font-weight: 800;
            color: var(--nato-primary);
            margin-bottom: var(--space-xs);
        }

        .alphabet-phonetic {
            font-size: 1rem;
            font-weight: 600;
            color: var(--nato-secondary);
            margin-bottom: var(--space-xs);
        }

        .alphabet-pronunciation {
            font-size: 0.75rem;
            color: var(--text-secondary);
            font-style: italic;
        }

        /* Examples Section */
        .examples-section {
            background: var(--surface-elevated);
            border-radius: var(--radius-xl);
            padding: var(--space-2xl);
            margin-bottom: var(--space-2xl);
            box-shadow: var(--shadow-lg);
            border: 1px solid var(--border-color);
        }

        .examples-grid {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(350px, 1fr));
            gap: var(--space-lg);
        }

        .example-card {
            background: linear-gradient(135deg, var(--aviation-silver), #ffffff);
            border: 2px solid var(--aviation-orange);
            border-radius: var(--radius-lg);
            padding: var(--space-lg);
            cursor: pointer;
            transition: var(--transition);
            position: relative;
            overflow: hidden;
        }

        .example-card::after {
            content: '';
            position: absolute;
            top: 0;
            right: 0;
            width: 0;
            height: 0;
            border-style: solid;
            border-width: 0 20px 20px 0;
            border-color: transparent var(--nato-primary) transparent transparent;
            opacity: 0;
            transition: var(--transition);
        }

        .example-card:hover::after {
            opacity: 1;
        }

        .example-card:hover {
            transform: translateY(-4px);
            box-shadow: var(--shadow-lg);
            border-color: var(--nato-primary);
        }

        .example-title {
            font-weight: 700;
            color: var(--nato-primary);
            margin-bottom: var(--space-sm);
            display: flex;
            align-items: center;
            gap: var(--space-sm);
        }

        .example-phonetic {
            font-family: 'Courier New', monospace;
            background: rgba(30, 58, 138, 0.1);
            padding: var(--space-md);
            border-radius: var(--radius-sm);
            margin: var(--space-sm) 0;
            font-size: 1rem;
            line-height: 1.4;
            color: var(--nato-primary);
            border: 1px dashed var(--nato-secondary);
        }

        .example-note {
            font-size: 0.875rem;
            color: var(--text-secondary);
            font-style: italic;
            display: flex;
            align-items: center;
            gap: var(--space-xs);
        }

        /* Practice Mode Section */
        .practice-section {
            background: linear-gradient(135deg, var(--military-green), var(--nato-green));
            border-radius: var(--radius-xl);
            padding: var(--space-2xl);
            margin-bottom: var(--space-2xl);
            color: white;
            box-shadow: var(--shadow-lg);
        }

        .practice-challenge {
            background: rgba(255, 255, 255, 0.1);
            border-radius: var(--radius-lg);
            padding: var(--space-xl);
            margin-top: var(--space-lg);
            backdrop-filter: blur(10px);
            border: 1px solid rgba(255, 255, 255, 0.2);
        }

        .challenge-letter {
            font-size: 4rem;
            font-weight: 800;
            text-align: center;
            margin-bottom: var(--space-lg);
            text-shadow: 2px 2px 8px rgba(0, 0, 0, 0.3);
        }

        .challenge-options {
            display: grid;
            grid-template-columns: repeat(2, 1fr);
            gap: var(--space-md);
        }