Bootstrap/less/labels.less

//
// Labels
// --------------------------------------------------

// LESS base

.label() {
  display: inline;
  padding: .25em .6em;
  font-size: 75%;
  font-weight: 500;
  color: #fff;
  line-height: 1;
  vertical-align: middle;
  white-space: nowrap;
  text-align: center;
  background-color: @gray-light;
  border-radius: .25em;

  // Hover state, but only for links - as a mixin which will be accessible as LESS shorthand: .label > .a;
  .a() {
    &:hover,
    &:focus {
      color: #fff;
      text-decoration: none;
      cursor: pointer;
    }
  }

  // Colors
  // Only give background-color difference to links (and to simplify, we don't qualifty with `a` but [href] attribute)
  // If there is a need for [href] then use local mixin a() via ex: .label-danger > .a; to attach additional CSS for [href] attr
  
  .label-danger() {
    background-color: @label-danger-bg;
    .a() {
      &[href] {
       background-color: darken(@label-danger-bg, 10%);
      }
    }
  }

  .label-warning() {
    background-color: @label-warning-bg;
    .a() {
      &[href] {
        background-color: darken(@label-warning-bg, 10%);
      }
    }
  }

  .label-success() {
    background-color: @label-success-bg;
    .a() {
      &[href] {
        background-color: darken(@label-success-bg, 10%);
      }
    }
  }

  .label-info() {
    background-color: @label-info-bg;
    .a() {
      &[href] {
        background-color: darken(@label-info-bg, 10%);
      }
    }
  }
}

// populate mixins for CSS
.label {
  .label();
}

a.label {
  .label > .a;
}

.label-danger {
  .label > .label-danger;
  .label > .label-danger > .a; // will produce .label-danger[href] class for folks who like to use class in HTML
}

.label-warning {
  .label > .label-warning;
  .label > .label-warning > .a;
}

.label-success {
  .label > .label-success;
  .label > .label-success > .a;
}

.label-info {
  .label > .label-info;
  .label > .label-info > .a;
}