twbs--bootstrap/scss/mixins/_hover.scss

@mixin hover {
  @if $enable-hover-media-query {
    // See Media Queries Level 4: http://drafts.csswg.org/mediaqueries/#hover
    // Currently shimmed by https://github.com/twbs/mq4-hover-shim
    @media (hover: hover) {
      &:hover { @content }
    }
  }
  @else {
    &:hover { @content }
  }
}

@mixin hover-focus {
  @if $enable-hover-media-query {
    &:focus { @content }
    @include hover { @content }
  }
  @else {
    &:focus,
    &:hover {
      @content
    }
  }
}

@mixin plain-hover-focus {
  @if $enable-hover-media-query {
    &,
    &:focus {
      @content
    }
    @include hover { @content }
  }
  @else {
    &,
    &:focus,
    &:hover {
      @content
    }
  }
}

@mixin hover-focus-active {
  @if $enable-hover-media-query {
    &:focus,
    &:active {
      @content
    }
    @include hover { @content }
  }
  @else {
    &:focus,
    &:active,
    &:hover {
      @content
    }
  }
}