:root {
            --primary-color: #044D8C;
            --accent-red: #F22233;
            --accent-gold: #A68F49;
            --accent-yellow: #F2BE5C;

            /* NTT Shades and Tints */
            --primary-tint-10: #1c558f;
            --primary-tint-80: #cdd9e6;
            --primary-tint-90: #e6ecf3;
            --primary-tint-95: #f2f6f9;
            --primary-shade-10: #033b75;
            --primary-shade-20: #023568;

            --radius: 8px;

            --bg-color: #F2F2F2;
            --card-bg: #ffffff;
            --text-color: #334155;
            --border-color: #e2e8f0;
            --success-color: #10b981;
            --danger-color: var(--accent-red);
            --ring-color: rgba(4, 77, 140, 0.15);
        }

        * {
            box-sizing: border-box;
            margin: 0;
            padding: 0;
            font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", system-ui, sans-serif;
        }

        body {
            background-color: var(--bg-color);
            color: var(--text-color);
            line-height: 1.6;
            -webkit-font-smoothing: antialiased;
        }

        header {
            display: flex;
            align-items: center;
            justify-content: center;
            gap: 15px;
            background-color: #ffffff;
            color: var(--primary-color);
            padding: 1.5rem 1rem;
            border-bottom: 1px solid var(--border-color);
            box-shadow: 0 1px 3px rgba(0, 0, 0, 0.05);
            margin-bottom: 2rem;
        }

        header h1 {
            font-size: 1.8rem;
            margin-bottom: 0;
            font-weight: 700;
            letter-spacing: -0.02em;
        }

        .container {
            max-width: 1000px;
            margin: 2rem auto;
            padding: 0 1rem;
        }

        .intro {
            margin-bottom: 2rem;
        }

        .intro p {
            margin-bottom: 1rem;
        }

        .intro ul {
            list-style: none;
        }

        .intro ul li {
            padding: 0.2rem 0;
        }

        .step-card {
            background: var(--card-bg);
            border: 1px solid var(--border-color);
            border-radius: 12px;
            padding: 2rem;
            margin-bottom: 2rem;
            box-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.05), 0 2px 4px -1px rgba(0, 0, 0, 0.03);
        }

        .step-title {
            color: var(--primary-color);
            font-size: 1.25rem;
            font-weight: 700;
            margin-top: 0;
            margin-bottom: 1.5rem;
            padding-bottom: 0.75rem;
            border-bottom: 1px solid var(--border-color);
            letter-spacing: -0.01em;
        }

        .form-group {
            margin-bottom: 1rem;
        }

        .form-group label {
            display: block;
            margin-bottom: 0.5rem;
        }

        .form-group select,
        .form-group input[type="number"] {
            width: 100%;
            padding: 10px 8px;
            border: 1px solid var(--border-color);
            border-radius: var(--radius);
            font-size: 16px;
            /* Ngăn iOS auto-zoom */
            -webkit-appearance: none;
            appearance: none;
            background-color: #fff;
            transition: border-color 0.2s, box-shadow 0.2s;
            color: var(--text-color);
        }

        .form-group select {
            background-image: url("data:image/svg+xml;charset=US-ASCII,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%22292.4%22%20height%3D%22292.4%22%3E%3Cpath%20fill%3D%22%23334155%22%20d%3D%22M287%2069.4a17.6%2017.6%200%200%200-13-5.4H18.4c-5%200-9.3%201.8-12.9%205.4A17.6%2017.6%200%200%200%200%2082.2c0%205%201.8%209.3%205.4%2012.9l128%20127.9c3.6%203.6%207.8%205.4%2012.8%205.4s9.2-1.8%2012.8-5.4L287%2095c3.5-3.5%205.4-7.8%205.4-12.8%200-5-1.9-9.2-5.5-12.8z%22%2F%3E%3C%2Fsvg%3E");
            background-repeat: no-repeat;
            background-position: right 0.75rem top 50%;
            background-size: 0.65rem auto;
            padding-right: 2rem;
        }

        .form-group select:focus,
        .form-group input[type="number"]:focus {
            outline: none;
            border-color: var(--primary-color);
            box-shadow: 0 0 0 3px var(--ring-color);
        }

        table {
            width: 100%;
            border-collapse: collapse;
        }

        .table-responsive {
            width: 100%;
            overflow-x: auto;
            -webkit-overflow-scrolling: touch;
            margin-bottom: 1.5rem;
        }

        @media (max-width: 768px) {
            header {
                flex-direction: column;
                gap: 10px;
                padding: 1rem;
            }

            .grid-2 {
                grid-template-columns: 1fr;
            }

            header h1 {
                font-size: 1.4rem;
                text-align: center;
            }

            th,
            td {
                padding: 8px 4px;
                font-size: 0.85rem;
            }

            .subject-name {
                min-width: 90px;
                word-wrap: break-word;
                white-space: normal;
            }

            td input {
                padding: 6px;
                min-width: 60px;
            }

            #result-score {
                font-size: 2rem;
            }

            .step-title {
                font-size: 1.1rem;
            }

            .formula-box {
                padding: 1rem;
            }

            .container {
                margin: 1rem auto;
            }

            .btn-submit {
                width: 100%;
                padding: 14px;
            }

            .table-responsive {
                border: 1px solid var(--border-color);
                border-radius: var(--radius);
            }
        }

        th,
        td {
            border: 1px solid var(--border-color);
            padding: 12px 10px;
            text-align: center;
        }

        th {
            background-color: var(--bg-color);
            font-weight: 600;
            color: var(--text-color);
        }

        td input {
            width: 100%;
            padding: 10px 6px;
            border: 1px solid var(--border-color);
            border-radius: var(--radius);
            text-align: center;
            font-size: 16px;
            /* Ngăn iOS auto-zoom */
            -webkit-appearance: none;
            appearance: none;
            background-color: #fff;
            transition: border-color 0.2s, box-shadow 0.2s;
            color: var(--text-color);
        }

        td input:focus {
            outline: none;
            border-color: var(--primary-color);
            box-shadow: 0 0 0 3px var(--ring-color);
        }

        .subject-name {
            text-align: center;
            font-weight: normal;
        }

        .table-title {
            font-weight: bold;
            margin-bottom: 0.5rem;
            margin-top: 1.5rem;
        }

        .grid-2 {
            display: grid;
            grid-template-columns: 1fr 1fr;
            gap: 1.5rem;
        }

        .actions {
            text-align: center;
            margin-top: 2rem;
        }

        .btn-submit {
            background-color: var(--primary-color);
            color: white;
            border: none;
            padding: 12px 32px;
            font-size: 16px;
            font-weight: 600;
            border-radius: var(--radius);
            cursor: pointer;
            transition: background-color 0.2s, transform 0.1s, box-shadow 0.2s;
            box-shadow: 0 2px 4px rgba(4, 77, 140, 0.2);
        }

        .btn-submit:hover {
            background-color: var(--accent-gold);
            box-shadow: 0 4px 6px rgba(166, 143, 73, 0.3);
            transform: translateY(-1px);
        }

        .btn-submit:active {
            transform: scale(0.98) translateY(0);
            box-shadow: 0 1px 2px rgba(3, 66, 130, 0.2);
        }

        #result-box {
            display: none;
            background-color: var(--card-bg);
            border: 1px solid var(--border-color);
            padding: 2.5rem;
            border-radius: 12px;
            text-align: center;
            margin-top: 2rem;
            box-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.05), 0 2px 4px -1px rgba(0, 0, 0, 0.03);
        }

        #result-box.pass {
            background-color: #e6f4ea;
            border-color: var(--success-color);
        }

        #result-box.fail {
            background-color: #fce8e6;
            border-color: var(--danger-color);
        }

        #result-score {
            font-size: 2.5rem;
            font-weight: bold;
            margin: 1rem 0;
        }

        .formula-section {
            margin-top: 3rem;
            padding-top: 2rem;
            border-top: 1px solid var(--border-color);
        }

        .formula-box {
            background-color: var(--card-bg);
            border: 1px solid var(--border-color);
            padding: 1.5rem;
            margin: 1.5rem 0;
            text-align: center;
            border-radius: 12px;
            box-shadow: 0 1px 3px rgba(0, 0, 0, 0.05);
        }

        .formula-box img {
            max-width: 100%;
            height: auto;
        }

        .header-logo {
            max-height: 65px;
            margin-bottom: 0;
        }

        #result-details {
            margin-top: 15px;
            font-size: 0.95rem;
            color: #555;
            text-align: left;
        }

        #form-error {
            display: none;
            color: var(--danger-color);
            margin-bottom: 1rem;
            font-weight: bold;
            text-align: center;
        }

        footer {
            background-color: #ffffff;
            color: var(--text-color);
            text-align: center;
            padding: 2rem 1.5rem;
            margin-top: 4rem;
            border-top: 1px solid var(--border-color);
            font-size: 0.9rem;
        }

        .input-method-selector {
            display: flex;
            gap: 12px;
            margin-bottom: 1.5rem;
        }

        .method-btn {
            flex: 1;
            padding: 12px;
            text-align: center;
            border: 1px solid var(--border-color);
            background-color: var(--card-bg);
            border-radius: var(--radius);
            cursor: pointer;
            font-weight: 600;
            transition: all 0.2s ease;
            color: var(--text-color);
        }

        .method-btn:hover {
            background-color: var(--primary-tint-95);
            border-color: var(--primary-color);
        }

        .method-btn.active {
            border-color: var(--primary-color);
            background-color: var(--primary-tint-95);
            color: var(--primary-color);
            box-shadow: 0 0 0 2px var(--ring-color);
        }

        .grid-3 {
            display: grid;
            grid-template-columns: repeat(3, 1fr);
            gap: 1.5rem;
        }

        @media (max-width: 768px) {
            .grid-3 {
                grid-template-columns: 1fr;
                gap: 1rem;
            }

            .input-method-selector {
                flex-direction: column;
                gap: 8px;
            }
        }

        #back-to-top {
            position: fixed;
            bottom: 30px;
            right: 30px;
            width: 45px;
            height: 45px;
            border-radius: 50%;
            background-color: var(--primary-color);
            color: white;
            border: none;
            font-size: 24px;
            cursor: pointer;
            display: none;
            align-items: center;
            justify-content: center;
            box-shadow: 0 4px 10px rgba(0, 0, 0, 0.15);
            transition: all 0.3s ease;
            z-index: 1000;
        }

        #back-to-top:hover {
            background-color: var(--primary-tint-10);
            transform: translateY(-3px);
            box-shadow: 0 6px 15px rgba(0, 0, 0, 0.2);
        }