Liquid Fill

:root {
  --ink: #050607;
  --acid: #c8ff3d;
}

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

body {
  min-height: 100vh;
  display: grid;
  place-items: center;
  background: var(--ink);
  font-family: "Segoe UI", system-ui, sans-serif;
}

.btn-liquid {
  position: relative;
  padding: 1rem 2.4rem;
  border: 2px solid var(--acid);
  border-radius: 999px;
  background: transparent;
  color: var(--acid);
  font-weight: 800;
  font-size: 0.95rem;
  cursor: pointer;
  overflow: hidden;
  transition: color 0.35s ease 0.05s;
}

.btn-liquid::before {
  content: "";
  position: absolute;
  left: 50%;
  top: 100%;
  width: 140%;
  height: 220%;
  background: var(--acid);
  border-radius: 42%;
  transform: translateX(-50%);
  transition: top 0.55s cubic-bezier(0.22, 1, 0.36, 1);
  z-index: 0;
}

.btn-liquid:hover {
  color: #06100a;
}

.btn-liquid:hover::before {
  top: -60%;
}

.btn-liquid span {
  position: relative;
  z-index: 1;
}