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

/* Mobile-first base styles */
body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
    background: linear-gradient(135deg, #1a1a1a 0%, #4a4a4a 100%);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 15px;
}

.container {
    max-width: 800px;
    width: 100%;
}

.header {
    text-align: center;
    color: white;
    margin-bottom: 20px;
}

.header h1 {
    font-size: 1.75rem;
    margin-bottom: 8px;
}

.subject-tabs {
    display: flex;
    gap: 8px;
    justify-content: center;
    margin-bottom: 20px;
    flex-wrap: wrap;
}

.tab {
    background: rgba(255, 255, 255, 0.2);
    color: white;
    border: 2px solid white;
    padding: 10px 20px;
    border-radius: 25px;
    cursor: pointer;
    font-size: 0.9rem;
    font-weight: 600;
    transition: all 0.3s;
    white-space: nowrap;
}

.tab:hover {
    background: rgba(255, 255, 255, 0.3);
}

.tab.active {
    background: white;
    color: #1a1a1a;
}

.card-container {
    perspective: 1000px;
    margin-bottom: 20px;
}

.card {
    background: white;
    border-radius: 15px;
    padding: 30px 20px;
    min-height: 350px;
    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.3);
    cursor: pointer;
    transition: transform 0.6s;
    transform-style: preserve-3d;
    position: relative;
}

.card.flipped {
    transform: rotateY(180deg);
}

.card-face {
    backface-visibility: hidden;
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    padding: 30px 20px;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    text-align: center;
    overflow-y: auto;
}

.card-back {
    transform: rotateY(180deg);
}

.card h2 {
    font-size: 1.3rem;
    color: #333;
    margin-bottom: 15px;
}

.card p, .card ul {
    font-size: 0.95rem;
    color: #666;
    line-height: 1.6;
    text-align: left;
}

.card ul {
    list-style-position: inside;
}

.card li {
    margin-bottom: 6px;
}

.controls {
    display: flex;
    justify-content: space-between;
    align-items: center;
    gap: 10px;
}

.btn {
    background: white;
    color: #1a1a1a;
    border: none;
    padding: 12px 20px;
    border-radius: 25px;
    font-size: 0.9rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.3s;
    box-shadow: 0 5px 15px rgba(0, 0, 0, 0.2);
}

.btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 8px 20px rgba(0, 0, 0, 0.3);
}

.btn:active {
    transform: translateY(0);
}

.btn:disabled {
    opacity: 0.5;
    cursor: not-allowed;
}

.progress {
    color: white;
    font-size: 1rem;
    font-weight: 600;
}

.shuffle-btn {
    background: rgba(255, 255, 255, 0.2);
    color: white;
    border: 2px solid white;
    margin-top: 15px;
    width: 100%;
}

.hint {
    color: rgba(255, 255, 255, 0.8);
    margin-top: 8px;
    font-size: 0.8rem;
}

/* Tablet breakpoint - 600px and up */
@media (min-width: 600px) {
    body {
        padding: 20px;
    }

    .header {
        margin-bottom: 25px;
    }

    .header h1 {
        font-size: 2rem;
    }

    .subject-tabs {
        gap: 10px;
        margin-bottom: 25px;
    }

    .tab {
        padding: 12px 25px;
        font-size: 0.95rem;
    }

    .card {
        border-radius: 18px;
        padding: 40px 30px;
        min-height: 380px;
    }

    .card-face {
        padding: 40px 30px;
    }

    .card h2 {
        font-size: 1.5rem;
        margin-bottom: 18px;
    }

    .card p, .card ul {
        font-size: 1rem;
        line-height: 1.7;
    }

    .card li {
        margin-bottom: 7px;
    }

    .btn {
        padding: 13px 25px;
        font-size: 0.95rem;
    }

    .progress {
        font-size: 1.1rem;
    }

    .shuffle-btn {
        width: auto;
        margin-top: 18px;
    }

    .hint {
        font-size: 0.85rem;
    }
}

/* Desktop breakpoint - 900px and up */
@media (min-width: 900px) {
    body {
        padding: 30px;
    }

    .header {
        margin-bottom: 30px;
    }

    .header h1 {
        font-size: 2.5rem;
        margin-bottom: 10px;
    }

    .subject-tabs {
        margin-bottom: 30px;
    }

    .tab {
        padding: 12px 30px;
        font-size: 1rem;
    }

    .card-container {
        margin-bottom: 30px;
    }

    .card {
        border-radius: 20px;
        padding: 60px 40px;
        min-height: 400px;
    }

    .card-face {
        padding: 60px 40px;
    }

    .card h2 {
        font-size: 1.8rem;
        margin-bottom: 20px;
    }

    .card p, .card ul {
        font-size: 1.1rem;
        line-height: 1.8;
    }

    .card li {
        margin-bottom: 8px;
    }

    .controls {
        gap: 20px;
    }

    .btn {
        padding: 15px 30px;
        font-size: 1rem;
    }

    .progress {
        font-size: 1.2rem;
    }

    .shuffle-btn {
        margin-top: 20px;
    }

    .hint {
        font-size: 0.9rem;
        margin-top: 10px;
    }
}

code {
    background: #f4f4f4;
    padding: 2px 6px;
    border-radius: 3px;
    font-family: 'Courier New', monospace;
}