* {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }
        
        :root {
            --primary-color: #667eea;
            --secondary-color: #764ba2;
            --accent-color: #ff6b6b;
            --text-dark: #2c3e50;
            --text-light: #666;
            --bg-light: #f8f9fa;
            --white: #ffffff;
        }
        
        body {
            font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
            line-height: 1.7;
            color: var(--text-dark);
            background: var(--white);
            overflow-x: hidden;
        }
        
        .container {
            max-width: 1200px;
            margin: 0 auto;
            padding: 0 20px;
        }
        
        header {
            background: rgba(255, 255, 255, 0.98);
            padding: 20px 0;
            box-shadow: 0 2px 20px rgba(0,0,0,0.08);
            position: sticky;
            top: 0;
            z-index: 1000;
            backdrop-filter: blur(10px);
        }
        
        .header-content {
            display: flex;
            justify-content: space-between;
            align-items: center;
            flex-wrap: wrap;
        }
        
        .logo {
            font-size: 24px;
            font-weight: 700;
            color: var(--primary-color);
            text-decoration: none;
        }
        
        .language-selector {
            position: relative;
        }
        
        .lang-btn {
            background: var(--bg-light);
            border: none;
            padding: 10px 20px;
            border-radius: 8px;
            cursor: pointer;
            font-size: 16px;
            display: flex;
            align-items: center;
            gap: 8px;
        }
        
        .lang-dropdown {
            display: none;
            position: absolute;
            top: 100%;
            right: 0;
            background: white;
            box-shadow: 0 4px 20px rgba(0,0,0,0.15);
            border-radius: 8px;
            margin-top: 8px;
            min-width: 200px;
            z-index: 1000;
        }
        
        .lang-dropdown.active {
            display: block;
        }
        
        .lang-dropdown a {
            display: block;
            padding: 12px 20px;
            color: var(--text-dark);
            text-decoration: none;
            transition: background 0.2s;
        }
        
        .lang-dropdown a:hover {
            background: var(--bg-light);
        }
        
        .breadcrumb {
            background: var(--bg-light);
            padding: 16px 0;
            font-size: 14px;
            color: var(--text-light);
        }
        
        .breadcrumb a {
            color: var(--primary-color);
            text-decoration: none;
        }
        
        .breadcrumb a:hover {
            text-decoration: underline;
        }
        
        .hero {
            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
            color: white;
            padding: 120px 20px 80px;
            text-align: center;
        }
        
        .hero h1 {
            font-size: 56px;
            font-weight: 800;
            margin-bottom: 24px;
            text-shadow: 0 4px 20px rgba(0,0,0,0.2);
            line-height: 1.2;
        }
        
        .hero-subtitle {
            font-size: 22px;
            margin-bottom: 16px;
            opacity: 0.95;
            font-weight: 300;
        }
        
        .hero-description {
            font-size: 18px;
            margin-bottom: 40px;
            max-width: 800px;
            margin-left: auto;
            margin-right: auto;
            opacity: 0.9;
            line-height: 1.8;
        }
        
        .cta-button {
            display: inline-block;
            background: var(--accent-color);
            color: white;
            padding: 18px 48px;
            border-radius: 50px;
            text-decoration: none;
            font-size: 18px;
            font-weight: 600;
            transition: all 0.3s ease;
            box-shadow: 0 8px 24px rgba(255, 107, 107, 0.4);
            position: relative;
            overflow: hidden;
        }
        
        .cta-button:hover {
            transform: translateY(-3px);
            box-shadow: 0 12px 32px rgba(255, 107, 107, 0.5);
        }
        
        .section {
            padding: 100px 20px;
        }
        
        .section-title {
            text-align: center;
            font-size: 42px;
            font-weight: 700;
            margin-bottom: 16px;
            color: var(--text-dark);
            line-height: 1.2;
        }
        
        .section-subtitle {
            text-align: center;
            font-size: 20px;
            color: var(--text-light);
            max-width: 700px;
            margin: 0 auto 60px;
            line-height: 1.6;
        }
        
        .content-section {
            background: var(--bg-light);
        }
        
        .content-block {
            background: white;
            padding: 50px;
            border-radius: 16px;
            box-shadow: 0 4px 20px rgba(0,0,0,0.08);
            margin-bottom: 40px;
        }
        
        .content-block h2 {
            font-size: 32px;
            font-weight: 700;
            margin-bottom: 24px;
            color: var(--text-dark);
            border-bottom: 3px solid var(--primary-color);
            padding-bottom: 16px;
        }
        
        .content-block h3 {
            font-size: 24px;
            font-weight: 600;
            margin-top: 32px;
            margin-bottom: 16px;
            color: var(--text-dark);
        }
        
        .content-block p {
            font-size: 18px;
            line-height: 1.8;
            margin-bottom: 20px;
            color: var(--text-light);
        }
        
        .content-block ul, .content-block ol {
            margin-left: 30px;
            margin-bottom: 20px;
        }
        
        .content-block li {
            font-size: 18px;
            line-height: 1.8;
            margin-bottom: 12px;
            color: var(--text-light);
        }
        
        .formula-box {
            background: var(--bg-light);
            border-left: 4px solid var(--primary-color);
            padding: 24px;
            margin: 24px 0;
            border-radius: 8px;
            font-family: 'Courier New', monospace;
        }
        
        .formula-box code {
            font-size: 16px;
            color: var(--text-dark);
        }
        
        .highlight-box {
            background: linear-gradient(135deg, rgba(102, 126, 234, 0.1), rgba(118, 75, 162, 0.1));
            border-left: 4px solid var(--primary-color);
            padding: 24px;
            margin: 24px 0;
            border-radius: 8px;
        }
        
        .highlight-box strong {
            color: var(--primary-color);
        }
        
        .example-box {
            background: #f0f4ff;
            border: 2px solid var(--primary-color);
            border-radius: 12px;
            padding: 24px;
            margin: 24px 0;
        }
        
        .example-box h4 {
            color: var(--primary-color);
            font-size: 20px;
            margin-bottom: 16px;
        }
        
        .related-links {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
            gap: 24px;
            margin-top: 40px;
        }
        
        .related-link-card {
            background: white;
            padding: 24px;
            border-radius: 12px;
            box-shadow: 0 2px 12px rgba(0,0,0,0.08);
            transition: all 0.3s ease;
        }
        
        .related-link-card:hover {
            transform: translateY(-4px);
            box-shadow: 0 8px 24px rgba(0,0,0,0.15);
        }
        
        .related-link-card a {
            color: var(--primary-color);
            text-decoration: none;
            font-weight: 600;
            font-size: 18px;
        }
        
        .related-link-card a:hover {
            text-decoration: underline;
        }
        
        .related-link-card p {
            margin-top: 12px;
            font-size: 16px;
            color: var(--text-light);
        }
        
        footer {
            background: var(--text-dark);
            color: white;
            padding: 60px 20px;
            text-align: center;
        }
        
        footer p {
            margin-bottom: 16px;
            line-height: 1.8;
        }
        
        footer a {
            color: var(--primary-color);
            text-decoration: none;
        }
        
        footer a:hover {
            text-decoration: underline;
        }
        
        .article-meta {
            color: var(--text-light);
            font-size: 14px;
            margin-bottom: 20px;
            font-style: italic;
            border-bottom: 1px solid var(--bg-light);
            padding-bottom: 12px;
        }
        
        .faq-item {
            margin-bottom: 25px;
            padding: 20px;
            background: white;
            border-radius: 8px;
            box-shadow: 0 2px 8px rgba(0,0,0,0.05);
        }
        
        .faq-item h3 {
            color: var(--primary-color);
            font-size: 20px;
            margin-bottom: 12px;
        }
        
        .faq-item p {
            color: var(--text-light);
            line-height: 1.8;
        }
        
        @media (max-width: 768px) {
            .hero h1 {
                font-size: 36px;
            }
            
            .hero-subtitle {
                font-size: 18px;
            }
            
            .section-title {
                font-size: 32px;
            }
            
            .content-block {
                padding: 30px 20px;
            }
            
            .content-block h2 {
                font-size: 28px;
            }
            
            .content-block h3 {
                font-size: 20px;
            }
        }

        }

        .nav-toggle {
            display: none;
            border: 1px solid rgba(0,0,0,0.12);
            background: rgba(255,255,255,0.9);
            border-radius: 10px;
            width: 44px;
            height: 44px;
            align-items: center;
            justify-content: center;
            cursor: pointer;
            padding: 0;
            font-size: 0; /* icon via ::before */
            transition: transform 0.2s, background 0.2s, border-color 0.2s;
            -webkit-tap-highlight-color: transparent;
        }

        .nav-toggle:hover {
            transform: translateY(-1px);
            border-color: rgba(102, 126, 234, 0.35);
            background: rgba(102, 126, 234, 0.06);
        }

        .nav-toggle::before {
            content: '☰';
            font-size: 22px;
            line-height: 1;
            color: var(--text-dark);
        }

        .nav-toggle.active::before {
            content: '✕';
        }

        @media (max-width: 820px) {

            .lang-dropdown {
                /* override absolute dropdown that can go off-screen */
                position: fixed;
                left: 16px;
                right: 16px;
                top: 72px;
                max-height: calc(50vh - 24px);
                overflow-y: auto;
                z-index: 3003;
            }

            .lang-dropdown.active {
                display: block;
            }

            .header-content {
                gap: 12px;
            }

            .nav-toggle {
                display: inline-flex;
                flex: 0 0 auto;
            }

            /* hide nav links until opened (inline styles overridden) */
            .main-nav {
                display: none !important;
            }

            .main-nav.is-open {
                display: flex !important;
                position: fixed;
                left: 0;
                right: 0;
                top: 72px;
                height: 50vh;
                max-height: 50vh;
                background: rgba(255, 255, 255, 0.98);
                backdrop-filter: blur(18px);
                -webkit-backdrop-filter: blur(18px);
                flex-direction: column;
                flex-wrap: nowrap !important;
                overflow-x: hidden;
                align-items: stretch;
                gap: 8px;
                padding: 16px;
                overflow-y: auto;
                box-shadow: 0 10px 30px rgba(0,0,0,0.12);
                border-bottom: 1px solid rgba(0,0,0,0.08);
                z-index: 1999;
            }

            .main-nav.is-open a {
                width: 100%;
                white-space: normal;
                overflow-wrap: anywhere;
                word-break: break-word;
                padding: 12px 14px;
                border-radius: 10px;
                background: rgba(0,0,0,0.02);
            }

            .main-nav.is-open a:hover {
                background: rgba(102, 126, 234, 0.10);
            }
        }