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

body {
  font-family: 'Noto Sans', sans-serif;
  background: #fff;
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  padding: 20px;
}

.container {
  width: 100%;
  max-width: 500px;
  display: flex;
  flex-direction: column;
  gap: 30px;
}

.card-container {
  perspective: 1000px;
  height: 300px;
}

.card {
  width: 100%;
  height: 100%;
  position: relative;
  transform-style: preserve-3d;
  transition: transform 0.6s;
  cursor: pointer;
}

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

.card-front,
.card-back {
  position: absolute;
  width: 100%;
  height: 100%;
  backface-visibility: hidden;
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 40px;
  border-radius: 12px;
  border: 2px solid #000;
  background: #fff;
}

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

.question,
.answer {
  font-size: 20px;
  text-align: center;
  line-height: 1.6;
}

.controls {
  display: flex;
  flex-direction: column;
  gap: 15px;
}

.btn {
  padding: 15px 30px;
  font-size: 16px;
  font-family: 'Noto Sans', sans-serif;
  border: 2px solid #000;
  border-radius: 8px;
  background: #fff;
  cursor: pointer;
  transition: all 0.2s;
}

.btn:active {
  transform: scale(0.98);
}

.btn-flip {
  background: #000;
  color: #fff;
}

.rating-btns {
  display: flex;
  gap: 10px;
}

.btn-again {
  flex: 1;
  border-color: #e74c3c;
  color: #e74c3c;
}

.btn-good {
  flex: 1;
  border-color: #3498db;
  color: #3498db;
}

.btn-easy {
  flex: 1;
  border-color: #2ecc71;
  color: #2ecc71;
}

.progress {
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.progress-text {
  font-size: 14px;
  text-align: center;
  color: #666;
}

.progress-bar {
  width: 100%;
  height: 8px;
  background: #eee;
  border-radius: 4px;
  overflow: hidden;
}

.progress-fill {
  height: 100%;
  background: #000;
  transition: width 0.3s;
  width: 0%;
}

@media (max-width: 600px) {
  .card-container {
    height: 250px;
  }

  .question,
  .answer {
    font-size: 18px;
  }

  .rating-btns {
    flex-direction: column;
  }

  .btn {
    padding: 12px 20px;
    font-size: 14px;
  }
}