KNACSS/sass/components/_buttons.scss

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

$burger-color             : $gray-800 !default;
$burger-background        : transparent !default;
$burger-hover-background  : transparent !default;
$burger-size              : 2.6rem !default;
$burger-weight            : 5px !default; // size of stripes
$burger-padding           : 0 !default;

%btn {
  display: inline-block;
  padding: $spacer-small $spacer-small-plus;
  cursor: pointer;
  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 {
    padding: $spacer-tiny-plus $spacer-small;
    font-size: .8em;
  }

  &--big {
    padding: $spacer-small-plus $spacer-medium;
    font-size: 1.4em;
  }

  &--block {
    width: 100% !important;
    display: block;
  }

  &--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;
    }
  }
}

// nav "burger" button
// activate it with a JS toggle-class to .is-active
// recommended HTML : <button class="nav-button" type="button" role="button" aria-label="open/close navigation"><i></i></button>
// see doc : https://knacss.com/styleguide.html#buttons
.nav-button {
  padding: 0;
  background-color: transparent;
  outline: 0;
  border: 0;
  cursor: pointer;
  -webkit-tap-highlight-color: transparent;

  & > * {
    display: inline-flex;
    vertical-align: top;
    flex-direction: column;
    justify-content: space-between;
    align-items: stretch;
    height: $burger-size;
    width: $burger-size;
    padding: $burger-padding;
    background-color: $burger-background;
    background-image: linear-gradient($burger-color, $burger-color);
    background-position: center;
    background-repeat: no-repeat;
    background-origin: content-box;
    background-size: 100% $burger-weight;
    transition: .25s;
    transition-property: transform, background;
    will-change: transform, background;

    &::before,
    &::after {
      content: "";
      height: $burger-weight;
      background: $burger-color;
      transition: .25s;
      transition-property: transform, top;
      will-change: transform, top;
    }
  }

  &:hover {
    & > * {
      background-color: $burger-hover-background;
    }
  }

  &:focus {
    outline: 0;
  }

  &.is-active {
    & > * {
      background-image: none;
      justify-content: center;

      &::before {
        transform: translateY(50%) rotate3d(0,0,1,45deg);
      }

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