*,
*::before,
*::after {
  box-sizing: border-box;
}

body {
  margin: 0;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  gap: 2rem;
}

.btn {
  --btn-color: #e2e2e2;
  --accent-color: #0af;
  --border-size: 0.4rem;

  border: none;
  width: max-content;
  font-size: 2rem;
  padding: 1rem;
  background-color: var(--btn-color);
  position: relative;
}

.btn:focus {
  outline: none;
}

.btn::before {
  position: absolute;
  content: "";
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  z-index: -1;
}

.btn-border-pop::before {
  border: var(--border-size) solid var(--btn-color);
  transition: top, left, right, bottom, 0.2s ease-in-out;
}

.btn-border-pop:hover::before,
.btn-border-pop:focus::before {
  top: calc(-2 * var(--border-size));
  left: calc(-2 * var(--border-size));
  right: calc(-2 * var(--border-size));
  bottom: calc(-2 * var(--border-size));
}

.btn-background-slide::before {
  background-color: var(--accent-color);
  transform: scaleX(0);
  transition: transform 0.2s ease-in-out;
  transform-origin: left;
}

.btn-background-slide:hover::before,
.btn-background-slide:focus::before {
  transform: scaleX(1);
}

.btn-background-slide {
  z-index: 1;
  transition: color 0.2s ease-in-out;
}

.btn-background-slide:hover,
.btn-background-slide:focus {
  color: white;
}

.btn-background-circle::before {
  border-radius: 50%;
  background-color: var(--btn-color);
  transform: scale(1.5);
  transition: transform 0.2s ease-in-out;
}

.btn-background-circle:hover::before,
.btn-background-circle:focus::before {
  transform: scale(0);
}

.btn-background-circle {
  overflow: hidden;
  z-index: 1;
  background-color: var(--accent-color);
  transition: color 0.2s ease-in-out;
}

.btn-background-circle:hover,
.btn-background-circle:focus {
  color: white;
}

.btn-border-underline::before {
  top: unset;
  height: var(--border-size);
  background-color: var(--accent-color);
  z-index: unset;
  transform: scaleX(0);
  transition: transform 0.2s ease-in-out;
}

.btn-border-underline:hover::before,
.btn-border-underline:focus::before {
  transform: scaleX(1);
}