KNACSS/sass/components/buttons.scss

/* ----------------------------- */
/* ==Buttons                     */
/* ----------------------------- */
/* preferably use <button> for buttons !*/
/* use .btn-- or .button-- classes for variants */

%btn {
  display: inline-block;
  padding: $tiny-value $small-value;
  cursor: pointer;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
  transition: 0.25s;
  transition-property: box-shadow, background-color, color, border;
  text-align: center;
  vertical-align: middle;
  white-space: nowrap;
  text-decoration: none;
  color: $color-base;
  border: none;
  border-radius: $border-radius;
  background-color: $color-muted;
  font-family: inherit;
  font-size: inherit;
  line-height: 1;
}

.btn,
.button,
[type="button"],
button {
  @extend %btn;

  &:focus {
    -webkit-tap-highlight-color: transparent;
  }
}

.btn,
.button {
  @each $name, $background-color, $color, $border in $variants-list {
    &--#{$name} {
      @extend %btn;
      background-color: $background-color;
      color: $color;
      box-shadow: $border;

      &:active,
      &:focus,
      &:hover {
        background-color: darken( $background-color, 10% );
      }
    }
  }
}

// button state variants
.btn,
.button {
  &--small {
    font-size: $base-font-size - 0.4rem;
  }

  &--big {
    font-size: $base-font-size + 0.4rem;
  }

  &--block {
    width: 100% !important;
    display: block;
  }
  &.disabled,
  &--disabled {
    opacity: 0.5;
    cursor: not-allowed;
  }
  &--unstyled {
    padding: 0;
    border: none;
    text-align: left;
    background: none;
    border-radius: 0;
    box-shadow: none;
    -webkit-appearance: none;
    -moz-appearance: none;
    appearance: none;

    &:focus {
      box-shadow: none;
      outline: none;
    }
  }
}

// burger button
.button-burger {
  position: relative;
  height: 2.4rem;
  width: 2.8rem;
  background-color: transparent;
  background-image: linear-gradient($color-burger, $color-burger);
  background-position: center;
  background-repeat: no-repeat;
  background-size: 100% 5px;
  padding: 0;
  outline: 0;
  border: 0;
  color: #333;
  cursor: pointer;
  -webkit-tap-highlight-color: transparent;
  transition: .25s cubic-bezier(.17,.67,.89,1.4);
  transition-property: transform;
  will-change: transform;

  & > * {
    @extend .visually-hidden;
  }

  &::before,
  &::after {
    content: "";
    position: absolute;
    left: 0; right: 0;
    display: block;
    height: 5px;
    background: $color-burger;
    transition: .25s;
    transition-property: transform, top;
    will-change: transform, top;
  }

  &::before {
    top: 0;
  }

  &:after {
    top: calc(100% - 4px);
  }

  &:focus,
  &.is-active,
  &.js-active {
    background-image: none;

    &::before,
    &::after {
       top: 50%;
    }
    &::before {
      transform: translate3d(0,-50%,0) rotate3d(0,0,1,45deg);
    }
    &::after {
      transform: translate3d(0,-50%,0) rotate3d(0,0,1,-45deg);
    }
  }
}