Card Number Auto-Format Input

<!--
  Author: @emma-lake
  License: MIT
  Tags: #buttons #css #interface #opensource #codepeek
-->
<div class="card-num-input">
  <span class="card-num-input__icon">&#9702;</span>
  <input type="text" class="card-num-input__field" placeholder="1234  5678  9012  3456" maxlength="19" />
</div>

/*
 * Author: @emma-lake
 * License: MIT
 * Tags: #buttons #css #interface #opensource #codepeek
 */

.card-num-input {
  position: relative;
  display: flex;
  align-items: center;
  gap: 10px;
  width: 260px;
  padding: 12px 16px;
  border-radius: 10px;
  background: #17171d;
  border: 1px solid #2a2a33;
  overflow: hidden;
  transition: border-color 0.3s ease, box-shadow 0.3s ease;
}
.card-num-input:focus-within {
  border-color: #34d399;
  box-shadow: 0 0 0 4px rgba(52, 211, 153, 0.15);
}
.card-num-input::after {
  content: "";
  position: absolute;
  top: 0;
  bottom: 0;
  left: -40%;
  width: 30%;
  background: linear-gradient(100deg, transparent, rgba(52, 211, 153, 0.10), transparent);
  animation: card-shimmer 2.6s ease-in-out infinite;
  pointer-events: none;
}
@keyframes card-shimmer {
  0% { left: -40%; }
  100% { left: 140%; }
}
.card-num-input__icon {
  font-size: 18px;
  color: #34d399;
  transition: transform 0.3s cubic-bezier(0.34, 1.56, 0.64, 1);
  animation: card-icon-flip 3.4s ease-in-out infinite;
}
@keyframes card-icon-flip {
  0%, 40% { transform: rotateY(0deg); }
  50%, 90% { transform: rotateY(180deg); }
  100% { transform: rotateY(360deg); }
}
.card-num-input__field {
  flex: 1;
  border: none;
  outline: none;
  background: transparent;
  color: #f1f1f1;
  font-family: 'Courier New', monospace;
  font-size: 15px;
  letter-spacing: 0.05em;
}
.card-num-input__field::placeholder {
  color: #6b6b76;
  letter-spacing: 0.05em;
}