Bootstrap/scss/forms/_floating-labels.scss

// We use mixins instead of placeholders as placeholders would combine the selectors when @extend-ed

@mixin form-floating-active-input-styles() {
  padding-top: $form-floating-input-padding-t;
  padding-bottom: $form-floating-input-padding-b;
}

@mixin form-floating-active-label-styles() {
  opacity: $form-floating-label-opacity;
  transform: $form-floating-label-transform;
}

.form-floating {
  position: relative;

  > .form-control,
  > .form-control-plaintext,
  > .form-select {
    height: $form-floating-height;
    line-height: $form-floating-line-height;
  }

  > label {
    position: absolute;
    top: 0;
    left: 0;
    height: 100%; // allow textareas
    padding: $form-floating-padding-y $form-floating-padding-x;
    pointer-events: none;
    border: $input-border-width solid transparent; // Required for aligning label's text with the input as it affects inner box model
    transform-origin: 0 0;
    @include transition($form-floating-transition);
  }

  > .form-control,
  > .form-control-plaintext {
    padding: $form-floating-padding-y $form-floating-padding-x;

    &::placeholder {
      color: transparent;
    }

    &:focus,
    &:not(:placeholder-shown) {
      @include form-floating-active-input-styles();
    }

    // Duplicated because `:-webkit-autofill` invalidates other selectors when grouped
    &:-webkit-autofill {
      @include form-floating-active-input-styles();
    }
  }

  > .form-select {
    @include form-floating-active-input-styles();
  }

  > .form-control:focus,
  > .form-control:not(:placeholder-shown),
  > .form-control-plaintext,
  > .form-select {
    ~ label {
      @include form-floating-active-label-styles();
    }
  }

  // Duplicated because `:-webkit-autofill` invalidates other selectors when grouped
  > .form-control:-webkit-autofill {
    ~ label {
      @include form-floating-active-label-styles();
    }
  }

  > .form-control-plaintext {
    ~ label {
      border-width: $input-border-width 0; // Required to properly position label text - as explained above
    }
  }
}

// Todo in v6: Consider combining this with the .form-control-plaintext rules to reduce some CSS?
.form-floating-always {
  > .form-control {
    @include form-floating-active-input-styles();

    &::placeholder {
      color: $input-color;
    }
    &:focus::placeholder {
      color: $input-placeholder-color;
    }
  }

  > label {
    @include form-floating-active-label-styles();
  }
}