Outlined buttons have the same appearance on :active and :focus states

Created by: TiGR

Slightly modified example from the docs:

<div class="btn-group" data-toggle="buttons">
  <label class="btn btn-primary-outline">
    <input type="checkbox" checked="" autocomplete="off"> Checkbox 1
  </label>
  <label class="btn btn-primary">
    <input type="checkbox" autocomplete="off"> Checkbox 2
  </label>
</div>

Once you clicked the button you can't say what is its state actually until you move your mouse pointer away (or on mobile devices - tap outside of the buttons area). So, if user have tapped some checkbox he can't really say what state is it in.

Compare this behavior with not outilined buttons.