Liquid Button

<button class="liquid-button">Liquid Fill</button>

.liquid-button {
  position: relative;
  padding: 16px 32px;
  background: transparent;
  color: #667eea;
  border: 2px solid #667eea;
  border-radius: 50px;
  font-size: 16px;
  font-weight: 600;
  cursor: pointer;
  overflow: hidden;
  transition: color 0.3s ease;
  z-index: 1;
}
.liquid-button::before {
  content: '';
  position: absolute;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 0;
  background: linear-gradient(45deg, #667eea, #764ba2);
  transition: height 0.4s cubic-bezier(0.25, 0.8, 0.25, 1);
  z-index: -1;
  border-radius: 50px;
}
.liquid-button:hover::before {
  height: 100%;
}
.liquid-button:hover {
  color: white;
  border-color: #764ba2;
}