<!-- start Simple Custom CSS and JS -->
<style type="text/css">
/* ============================================
   OSI Model Webpage - 2025 Design Trends
   Features: Gradients, Animations, Dark Mode, Modern Typography
   Enhanced with Detailed Packet Flow Visualization
   ============================================ */

/* CSS Variables for Theme Colors */
:root {
    /* 2025 Color Palette - Mocha Mousse & Warm Tones */
    --primary-color: #6366f1;
    --primary-dark: #4f46e5;
    --secondary-color: #ec4899;
    --accent-color: #f59e0b;
    --mocha: #a67c52;
    --mocha-light: #c4a47c;
    
    /* Neutral Colors */
    --bg-color: #ffffff;
    --bg-secondary: #f9fafb;
    --text-primary: #111827;
    --text-secondary: #6b7280;
    --border-color: #e5e7eb;
    
    /* Layer Colors */
    --layer-7: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    --layer-6: linear-gradient(135deg, #f093fb 0%, #f5576c 100%);
    --layer-5: linear-gradient(135deg, #4facfe 0%, #00f2fe 100%);
    --layer-4: linear-gradient(135deg, #43e97b 0%, #38f9d7 100%);
    --layer-3: linear-gradient(135deg, #fa709a 0%, #fee140 100%);
    --layer-2: linear-gradient(135deg, #30cfd0 0%, #330867 100%);
    --layer-1: linear-gradient(135deg, #a8edea 0%, #fed6e3 100%);
    
    /* Spacing */
    --spacing-xs: 0.5rem;
    --spacing-sm: 1rem;
    --spacing-md: 2rem;
    --spacing-lg: 4rem;
    --spacing-xl: 6rem;
    
    /* Typography */
    --font-primary: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
    --font-heading: 'Space Grotesk', sans-serif;
    
    /* Shadows */
    --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.12);
    --shadow-md: 0 4px 6px rgba(0, 0, 0, 0.1);
    --shadow-lg: 0 10px 25px rgba(0, 0, 0, 0.15);
    --shadow-xl: 0 20px 40px rgba(0, 0, 0, 0.2);
    
    /* Border Radius */
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 20px;
    --radius-xl: 30px;
}

/* Dark Mode Variables */
[data-theme="dark"] {
    --bg-color: #0f172a;
    --bg-secondary: #1e293b;
    --text-primary: #f1f5f9;
    --text-secondary: #cbd5e1;
    --border-color: #334155;
}

/* Reset & Base Styles */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
    font-size: 16px;
}

body {
    font-family: var(--font-primary);
    background-color: var(--bg-color);
    color: var(--text-primary);
    line-height: 1.7;
    transition: background-color 0.3s ease, color 0.3s ease;
    overflow-x: hidden;
}

/* Container */
.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 var(--spacing-md);
}

/* ============================================
   HEADER / HERO SECTION
   ============================================ */
.hero-section {
    position: relative;
    min-height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    background: linear-gradient(135deg, #667eea 0%, #764ba2 50%, #f093fb 100%);
    overflow: hidden;
    padding: var(--spacing-lg) 0;
}

.hero-decoration {
    position: absolute;
    width: 100%;
    height: 100%;
    top: 0;
    left: 0;
    opacity: 0.1;
    background-image: 
        radial-gradient(circle at 20% 50%, white 2px, transparent 2px),
        radial-gradient(circle at 80% 80%, white 2px, transparent 2px);
    background-size: 100px 100px;
    animation: float 20s ease-in-out infinite;
}

@keyframes float {
    0%, 100% { transform: translateY(0px); }
    50% { transform: translateY(-20px); }
}

.hero-content {
    position: relative;
    z-index: 2;
    text-align: center;
    color: white;
    animation: fadeInUp 1s ease-out;
}

@keyframes fadeInUp {
    from {
        opacity: 0;
        transform: translateY(30px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.badge {
    display: inline-block;
    padding: var(--spacing-xs) var(--spacing-md);
    background: rgba(255, 255, 255, 0.2);
    backdrop-filter: blur(10px);
    border-radius: 50px;
    font-size: 0.875rem;
    font-weight: 600;
    letter-spacing: 0.5px;
    text-transform: uppercase;
    margin-bottom: var(--spacing-sm);
    border: 1px solid rgba(255, 255, 255, 0.3);
}

.hero-title {
    font-family: var(--font-heading);
    font-size: clamp(3rem, 8vw, 6rem);
    font-weight: 800;
    margin-bottom: var(--spacing-sm);
    line-height: 1.1;
    letter-spacing: -2px;
    text-shadow: 0 4px 20px rgba(0, 0, 0, 0.3);
}

.hero-subtitle {
    font-size: clamp(1.125rem, 2vw, 1.5rem);
    margin-bottom: var(--spacing-lg);
    opacity: 0.95;
    font-weight: 300;
}

.cta-buttons {
    display: flex;
    gap: var(--spacing-sm);
    justify-content: center;
    flex-wrap: wrap;
}

.btn {
    padding: 1rem 2rem;
    border: none;
    border-radius: var(--radius-md);
    font-family: var(--font-primary);
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.3s ease;
    display: inline-flex;
    align-items: center;
    gap: var(--spacing-xs);
    text-decoration: none;
}

.btn-primary {
    background: white;
    color: var(--primary-color);
    box-shadow: var(--shadow-lg);
}

.btn-primary:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-xl);
}

.btn-secondary {
    background: rgba(255, 255, 255, 0.15);
    color: white;
    border: 2px solid rgba(255, 255, 255, 0.3);
    backdrop-filter: blur(10px);
}

.btn-secondary:hover {
    background: rgba(255, 255, 255, 0.25);
    transform: translateY(-2px);
}

/* ============================================
   SECTION HEADERS
   ============================================ */
.section-header {
    text-align: center;
    margin-bottom: var(--spacing-lg);
    animation: fadeIn 0.8s ease-out;
}

@keyframes fadeIn {
    from { opacity: 0; }
    to { opacity: 1; }
}

.section-title {
    font-family: var(--font-heading);
    font-size: clamp(2rem, 5vw, 3rem);
    font-weight: 700;
    margin-bottom: var(--spacing-sm);
    color: var(--text-primary);
}

.title-underline {
    width: 80px;
    height: 4px;
    background: linear-gradient(90deg, var(--primary-color), var(--secondary-color));
    margin: 0 auto var(--spacing-sm);
    border-radius: 2px;
}

.section-description {
    font-size: 1.125rem;
    color: var(--text-secondary);
    max-width: 600px;
    margin: 0 auto;
}

/* ============================================
   DEFINITION SECTION
   ============================================ */
.definition-section {
    padding: var(--spacing-xl) 0;
    background: var(--bg-color);
}

.definition-content {
    max-width: 900px;
    margin: 0 auto;
}

.definition-card {
    background: var(--bg-secondary);
    padding: var(--spacing-lg);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-md);
    transition: transform 0.3s ease, box-shadow 0.3s ease;
    border: 1px solid var(--border-color);
}

.definition-card:hover {
    transform: translateY(-5px);
    box-shadow: var(--shadow-lg);
}

.icon-wrapper {
    text-align: center;
    margin-bottom: var(--spacing-md);
}

.icon-large {
    font-size: 4rem;
    display: inline-block;
    animation: pulse 2s ease-in-out infinite;
}

@keyframes pulse {
    0%, 100% { transform: scale(1); }
    50% { transform: scale(1.1); }
}

.definition-card p {
    font-size: 1.125rem;
    line-height: 1.8;
    margin-bottom: var(--spacing-sm);
}

.definition-card p:last-child {
    margin-bottom: 0;
}

/* ============================================
   COMPARISON SECTION
   ============================================ */
.comparison-section {
    padding: var(--spacing-xl) 0;
    background: var(--bg-secondary);
}

.comparison-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: var(--spacing-lg);
    max-width: 1000px;
    margin: 0 auto;
}

.comparison-card {
    background: var(--bg-color);
    padding: var(--spacing-lg);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-md);
    transition: all 0.3s ease;
    border: 2px solid transparent;
    position: relative;
    overflow: hidden;
}

.comparison-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 4px;
    background: linear-gradient(90deg, var(--primary-color), var(--secondary-color));
    transform: scaleX(0);
    transition: transform 0.3s ease;
}

.comparison-card:hover::before {
    transform: scaleX(1);
}

.comparison-card:hover {
    transform: translateY(-8px);
    box-shadow: var(--shadow-xl);
    border-color: var(--primary-color);
}

.card-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: var(--spacing-md);
    padding-bottom: var(--spacing-sm);
    border-bottom: 2px solid var(--border-color);
}

.card-header h3 {
    font-family: var(--font-heading);
    font-size: 1.5rem;
    color: var(--text-primary);
}

.layer-count {
    background: linear-gradient(135deg, var(--primary-color), var(--secondary-color));
    color: white;
    padding: 0.5rem 1rem;
    border-radius: 50px;
    font-weight: 700;
    font-size: 0.875rem;
}

.comparison-list {
    list-style: none;
}

.comparison-list li {
    padding: var(--spacing-sm) 0;
    display: flex;
    align-items: flex-start;
    gap: var(--spacing-sm);
    font-size: 1rem;
    line-height: 1.6;
}

.check-icon {
    color: #10b981;
    font-weight: bold;
    font-size: 1.25rem;
    flex-shrink: 0;
}

/* ============================================
   OSI LAYERS SECTION
   ============================================ */
.layers-section {
    padding: var(--spacing-xl) 0;
    background: var(--bg-color);
}

.layers-stack {
    max-width: 900px;
    margin: 0 auto;
    display: flex;
    flex-direction: column;
    gap: var(--spacing-sm);
}

.layer-card {
    display: grid;
    grid-template-columns: 80px 1fr;
    gap: var(--spacing-md);
    padding: var(--spacing-md);
    border-radius: var(--radius-md);
    background: var(--bg-secondary);
    border: 2px solid var(--border-color);
    transition: all 0.4s cubic-bezier(0.4, 0, 0.2, 1);
    cursor: pointer;
    position: relative;
    overflow: hidden;
}

.layer-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    opacity: 0;
    transition: opacity 0.4s ease;
    z-index: 0;
}

.layer-card.layer-7::before { background: var(--layer-7); }
.layer-card.layer-6::before { background: var(--layer-6); }
.layer-card.layer-5::before { background: var(--layer-5); }
.layer-card.layer-4::before { background: var(--layer-4); }
.layer-card.layer-3::before { background: var(--layer-3); }
.layer-card.layer-2::before { background: var(--layer-2); }
.layer-card.layer-1::before { background: var(--layer-1); }

.layer-card:hover::before {
    opacity: 0.15;
}

.layer-card:hover {
    transform: translateX(10px) scale(1.02);
    box-shadow: var(--shadow-lg);
    border-color: transparent;
}

.layer-number {
    display: flex;
    align-items: center;
    justify-content: center;
    width: 60px;
    height: 60px;
    border-radius: 50%;
    font-family: var(--font-heading);
    font-size: 1.75rem;
    font-weight: 700;
    color: white;
    position: relative;
    z-index: 1;
    flex-shrink: 0;
}

.layer-7 .layer-number { background: var(--layer-7); }
.layer-6 .layer-number { background: var(--layer-6); }
.layer-5 .layer-number { background: var(--layer-5); }
.layer-4 .layer-number { background: var(--layer-4); }
.layer-3 .layer-number { background: var(--layer-3); }
.layer-2 .layer-number { background: var(--layer-2); }
.layer-1 .layer-number { background: var(--layer-1); }

.layer-info {
    position: relative;
    z-index: 1;
}

.layer-name {
    font-family: var(--font-heading);
    font-size: 1.5rem;
    margin-bottom: var(--spacing-xs);
    color: var(--text-primary);
}

.layer-description {
    font-size: 1rem;
    line-height: 1.6;
    color: var(--text-secondary);
    margin-bottom: var(--spacing-sm);
}

.layer-details {
    display: flex;
    flex-wrap: wrap;
    gap: var(--spacing-xs);
}

.detail-badge {
    padding: 0.25rem 0.75rem;
    background: rgba(99, 102, 241, 0.1);
    color: var(--primary-color);
    border-radius: 50px;
    font-size: 0.875rem;
    font-weight: 600;
    border: 1px solid rgba(99, 102, 241, 0.2);
}

/* ============================================
   ENHANCED DATA FLOW SECTION
   ============================================ */
.dataflow-section {
    padding: var(--spacing-xl) 0;
    background: var(--bg-secondary);
}

/* Overview Cards */
.dataflow-overview {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: var(--spacing-lg);
    margin-bottom: var(--spacing-xl);
}

.overview-card {
    background: var(--bg-color);
    padding: var(--spacing-lg);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-md);
    text-align: center;
    transition: all 0.3s ease;
    border: 2px solid var(--border-color);
}

.overview-card:hover {
    transform: translateY(-5px);
    box-shadow: var(--shadow-lg);
}

.overview-icon {
    font-size: 3rem;
    margin-bottom: var(--spacing-sm);
}

.overview-card h3 {
    font-family: var(--font-heading);
    font-size: 1.5rem;
    margin-bottom: var(--spacing-sm);
    color: var(--text-primary);
}

.overview-card p {
    font-size: 1rem;
    line-height: 1.6;
    color: var(--text-secondary);
}

/* Detailed Flow Section */
.detailed-flow {
    max-width: 1000px;
    margin: 0 auto;
}

.flow-subtitle {
    font-family: var(--font-heading);
    font-size: 1.75rem;
    text-align: center;
    margin-bottom: var(--spacing-lg);
    color: var(--text-primary);
}

/* Process Selector */
.process-selector {
    display: flex;
    gap: var(--spacing-sm);
    justify-content: center;
    margin-bottom: var(--spacing-lg);
    flex-wrap: wrap;
}

.process-btn {
    padding: 1rem 2rem;
    border: 2px solid var(--border-color);
    border-radius: var(--radius-md);
    background: var(--bg-color);
    color: var(--text-primary);
    font-family: var(--font-primary);
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.3s ease;
    display: inline-flex;
    align-items: center;
    gap: var(--spacing-xs);
}

.process-btn.active {
    background: linear-gradient(135deg, var(--primary-color), var(--secondary-color));
    color: white;
    border-color: transparent;
    box-shadow: var(--shadow-md);
}

.process-btn:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}

.btn-icon {
    font-size: 1.25rem;
}

/* Process Details */
.process-details {
    display: none;
}

.process-details.active {
    display: block;
    animation: fadeIn 0.5s ease;
}

/* Layer Process Cards */
.layer-process {
    background: var(--bg-color);
    border-radius: var(--radius-lg);
    margin-bottom: var(--spacing-md);
    overflow: hidden;
    box-shadow: var(--shadow-md);
    transition: all 0.3s ease;
    border: 2px solid var(--border-color);
}

.layer-process:hover {
    box-shadow: var(--shadow-lg);
    transform: translateX(5px);
}

.process-header {
    padding: var(--spacing-md);
    color: white;
    display: flex;
    align-items: center;
    justify-content: space-between;
    flex-wrap: wrap;
    gap: var(--spacing-sm);
}

.layer-7-bg { background: var(--layer-7); }
.layer-6-bg { background: var(--layer-6); }
.layer-5-bg { background: var(--layer-5); }
.layer-4-bg { background: var(--layer-4); }
.layer-3-bg { background: var(--layer-3); }
.layer-2-bg { background: var(--layer-2); }
.layer-1-bg { background: var(--layer-1); }

.layer-badge {
    background: rgba(255, 255, 255, 0.2);
    backdrop-filter: blur(10px);
    padding: 0.375rem 0.875rem;
    border-radius: 50px;
    font-size: 0.875rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.process-header h4 {
    font-family: var(--font-heading);
    font-size: 1.5rem;
    font-weight: 700;
    flex: 1;
    min-width: 200px;
}

.pdu-name {
    background: rgba(255, 255, 255, 0.25);
    backdrop-filter: blur(10px);
    padding: 0.375rem 1rem;
    border-radius: 50px;
    font-size: 0.875rem;
    font-weight: 600;
    border: 1px solid rgba(255, 255, 255, 0.3);
}

.process-content {
    padding: var(--spacing-md);
}

.process-step {
    display: flex;
    gap: var(--spacing-sm);
    margin-bottom: var(--spacing-md);
    align-items: flex-start;
}

.process-step:last-child {
    margin-bottom: 0;
}

.step-icon {
    font-size: 1.5rem;
    flex-shrink: 0;
    line-height: 1;
}

.process-step p {
    font-size: 1rem;
    line-height: 1.6;
    color: var(--text-primary);
}

.process-step strong {
    color: var(--primary-color);
    font-weight: 700;
}

.sub-list {
    list-style: none;
    margin-top: var(--spacing-xs);
    padding-left: var(--spacing-md);
}

.sub-list li {
    padding: 0.25rem 0;
    position: relative;
    color: var(--text-secondary);
    font-size: 0.95rem;
}

.sub-list li::before {
    content: '▸';
    position: absolute;
    left: -1rem;
    color: var(--primary-color);
}

/* Data Flow Summary */
.dataflow-summary {
    margin-top: var(--spacing-xl);
    padding: var(--spacing-lg);
    background: var(--bg-color);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-md);
}

.dataflow-summary h3 {
    font-family: var(--font-heading);
    font-size: 1.75rem;
    text-align: center;
    margin-bottom: var(--spacing-lg);
    color: var(--text-primary);
}

.summary-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: var(--spacing-md);
}

.summary-card {
    background: var(--bg-secondary);
    padding: var(--spacing-md);
    border-radius: var(--radius-md);
    text-align: center;
    border: 2px solid var(--border-color);
    transition: all 0.3s ease;
}

.summary-card:hover {
    transform: translateY(-3px);
    border-color: var(--primary-color);
}

.summary-icon {
    font-size: 2.5rem;
    margin-bottom: var(--spacing-sm);
    display: block;
}

.summary-card h4 {
    font-family: var(--font-heading);
    font-size: 1.125rem;
    margin-bottom: var(--spacing-xs);
    color: var(--text-primary);
}

.summary-card p {
    font-size: 0.95rem;
    line-height: 1.6;
    color: var(--text-secondary);
}

/* ============================================
   FOOTER
   ============================================ */
.footer {
    padding: var(--spacing-lg) 0;
    background: var(--bg-secondary);
    text-align: center;
    border-top: 1px solid var(--border-color);
}

.footer p {
    color: var(--text-secondary);
    font-size: 0.95rem;
}

/* ============================================
   RESPONSIVE DESIGN
   ============================================ */
@media (max-width: 768px) {
    .hero-title {
        font-size: 2.5rem;
    }
    
    .comparison-grid {
        grid-template-columns: 1fr;
    }
    
    .layer-card {
        grid-template-columns: 1fr;
        text-align: center;
    }
    
    .layer-number {
        margin: 0 auto var(--spacing-sm);
    }
    
    .layer-details {
        justify-content: center;
    }
    
    .process-header {
        text-align: center;
        justify-content: center;
    }
    
    .process-header h4 {
        min-width: auto;
        width: 100%;
        text-align: center;
    }
    
    .process-selector {
        flex-direction: column;
    }
    
    .process-btn {
        width: 100%;
        justify-content: center;
    }
}

/* ============================================
   SCROLL ANIMATIONS
   ============================================ */
.fade-in {
    opacity: 0;
    transform: translateY(30px);
    transition: opacity 0.6s ease, transform 0.6s ease;
}

.fade-in.visible {
    opacity: 1;
    transform: translateY(0);
}

/* ============================================
   ACCESSIBILITY
   ============================================ */
@media (prefers-reduced-motion: reduce) {
    *,
    *::before,
    *::after {
        animation-duration: 0.01ms !important;
        animation-iteration-count: 1 !important;
        transition-duration: 0.01ms !important;
    }
}

/* Focus Styles */
*:focus-visible {
    outline: 3px solid var(--primary-color);
    outline-offset: 3px;
}
</style>
<!-- end Simple Custom CSS and JS -->
